Mark some intentional fall-throughs (in a way understandable to gcc)

This silences -Wimplicit-fallthrough warnings with gcc7.

(Re-Apply without the unintended commits)

src/Paragraph.cpp

@@ -3871,13 +3871,13 @@ void Paragraph::locateWord(pos_type & from, pos_type & to,
 			to = from;
 			return;
 		}
-		// no break here, we go to the next
+		// fall through
 
 	case WHOLE_WORD:
 		// If we are already at the beginning of a word, do nothing
 		if (!from || isWordSeparator(from - 1))
 			break;
-		// no break here, we go to the next
+		// fall through
 
 	case PREVIOUS_WORD:
 		// always move the cursor to the beginning of previous word

src/frontends/qt4/GuiApplication.cpp

@@ -2070,9 +2070,8 @@ void GuiApplication::dispatch(FuncRequest const & cmd, DispatchResult & dr)
 			if (current_view_ == 0)
 				createView();
 		}
-		// fall through
 	}
-
+	// fall through
 	default:
 		// The LFUN must be for one of GuiView, BufferView, Buffer or Cursor;
 		// let's try that:

src/frontends/qt4/GuiView.cpp

@@ -1989,7 +1989,8 @@ bool GuiView::getStatus(FuncRequest const & cmd, FuncStatus & flag)
 
 	case LFUN_DIALOG_TOGGLE:
 		flag.setOnOff(isDialogVisible(cmd.getArg(0)));
-		// fall through to set "enable"
+		// to set "enable"
+		// fall through
 	case LFUN_DIALOG_SHOW: {
 		string const name = cmd.getArg(0);
 		if (!doc_buffer)

src/insets/InsetCaption.cpp

@@ -209,6 +209,8 @@ void InsetCaption::doDispatch(Cursor & cur, FuncRequest & cmd)
 			break;
 		}
 	}
+	// no "changetype":
+	// fall through
 
 	default:
 		InsetText::doDispatch(cur, cmd);

src/insets/InsetFloat.cpp

@@ -209,6 +209,8 @@ bool InsetFloat::getStatus(Cursor & cur, FuncRequest const & cmd,
 			flag.setEnabled(false);
 			return true;
 		}
+		// no subfloat:
+		// fall through
 
 	default:
 		return InsetCaptionable::getStatus(cur, cmd, flag);

src/insets/InsetLabel.cpp

@@ -214,6 +214,8 @@ bool InsetLabel::getStatus(Cursor & cur, FuncRequest const & cmd,
 			enabled = false;
 			break;
 		}
+		// no "changetype":
+		// fall through
 	default:
 		return InsetCommand::getStatus(cur, cmd, status);
 	}

src/insets/InsetListings.cpp

@@ -441,6 +441,7 @@ bool InsetListings::getStatus(Cursor & cur, FuncRequest const & cmd,
 				return true;
 			}
 		}
+		// fall through
 		default:
 			return InsetCaptionable::getStatus(cur, cmd, status);
 	}

src/insets/InsetTabular.cpp

@@ -4676,12 +4676,14 @@ bool InsetTabular::getFeatureStatus(Cursor & cur, string const & s,
 		// therefore allow always left but right and center only if there is no width
 		case Tabular::M_ALIGN_LEFT:
 			flag = false;
+			// fall through
 		case Tabular::ALIGN_LEFT:
 			status.setOnOff(tabular.getAlignment(cur.idx(), flag) == LYX_ALIGN_LEFT);
 			break;
 
 		case Tabular::M_ALIGN_RIGHT:
 			flag = false;
+			// fall through
 		case Tabular::ALIGN_RIGHT:
 			status.setEnabled(!(tabular.isMultiRow(cur.idx())
 				&& !tabular.getPWidth(cur.idx()).zero()));
@@ -4690,6 +4692,7 @@ bool InsetTabular::getFeatureStatus(Cursor & cur, string const & s,
 
 		case Tabular::M_ALIGN_CENTER:
 			flag = false;
+			// fall through
 		case Tabular::ALIGN_CENTER:
 			status.setEnabled(!(tabular.isMultiRow(cur.idx())
 				&& !tabular.getPWidth(cur.idx()).zero()));
@@ -4710,6 +4713,7 @@ bool InsetTabular::getFeatureStatus(Cursor & cur, string const & s,
 
 		case Tabular::M_VALIGN_TOP:
 			flag = false;
+			// fall through
 		case Tabular::VALIGN_TOP:
 			status.setEnabled(!tabular.getPWidth(cur.idx()).zero()
 				&& !tabular.isMultiRow(cur.idx()));
@@ -4719,6 +4723,7 @@ bool InsetTabular::getFeatureStatus(Cursor & cur, string const & s,
 
 		case Tabular::M_VALIGN_BOTTOM:
 			flag = false;
+			// fall through
 		case Tabular::VALIGN_BOTTOM:
 			status.setEnabled(!tabular.getPWidth(cur.idx()).zero()
 				&& !tabular.isMultiRow(cur.idx()));
@@ -4728,6 +4733,7 @@ bool InsetTabular::getFeatureStatus(Cursor & cur, string const & s,
 
 		case Tabular::M_VALIGN_MIDDLE:
 			flag = false;
+			// fall through
 		case Tabular::VALIGN_MIDDLE:
 			status.setEnabled(!tabular.getPWidth(cur.idx()).zero()
 				&& !tabular.isMultiRow(cur.idx()));
@@ -5006,7 +5012,7 @@ bool InsetTabular::getStatus(Cursor & cur, FuncRequest const & cmd,
 			status.setEnabled(false);
 			return true;
 		}
-		// Fall back
+		// fall through
 	case LFUN_NEWLINE_INSERT: {
 		if (tabular.getPWidth(cur.idx()).zero()) {
 			status.setEnabled(false);
@@ -5660,6 +5666,7 @@ void InsetTabular::tabularFeatures(Cursor & cur,
 	case Tabular::M_VALIGN_BOTTOM:
 	case Tabular::M_VALIGN_MIDDLE:
 		flag = false;
+		// fall through
 	case Tabular::VALIGN_TOP:
 	case Tabular::VALIGN_BOTTOM:
 	case Tabular::VALIGN_MIDDLE:
@@ -5782,6 +5789,7 @@ void InsetTabular::tabularFeatures(Cursor & cur,
 
 	case Tabular::SET_ALL_LINES:
 		setLines = true;
+		// fall through
 	case Tabular::UNSET_ALL_LINES:
 		for (row_type r = sel_row_start; r <= sel_row_end; ++r)
 			for (col_type c = sel_col_start; c <= sel_col_end; ++c) {
@@ -5906,6 +5914,7 @@ void InsetTabular::tabularFeatures(Cursor & cur,
 
 	case Tabular::UNSET_LTFIRSTHEAD:
 		flag = false;
+		// fall through
 	case Tabular::SET_LTFIRSTHEAD:
 		tabular.getRowOfLTFirstHead(row, ltt);
 		checkLongtableSpecial(ltt, value, flag);
@@ -5914,6 +5923,7 @@ void InsetTabular::tabularFeatures(Cursor & cur,
 
 	case Tabular::UNSET_LTHEAD:
 		flag = false;
+		// fall through
 	case Tabular::SET_LTHEAD:
 		tabular.getRowOfLTHead(row, ltt);
 		checkLongtableSpecial(ltt, value, flag);
@@ -5922,6 +5932,7 @@ void InsetTabular::tabularFeatures(Cursor & cur,
 
 	case Tabular::UNSET_LTFOOT:
 		flag = false;
+		// fall through
 	case Tabular::SET_LTFOOT:
 		tabular.getRowOfLTFoot(row, ltt);
 		checkLongtableSpecial(ltt, value, flag);
@@ -5930,6 +5941,7 @@ void InsetTabular::tabularFeatures(Cursor & cur,
 
 	case Tabular::UNSET_LTLASTFOOT:
 		flag = false;
+		// fall through
 	case Tabular::SET_LTLASTFOOT:
 		tabular.getRowOfLTLastFoot(row, ltt);
 		checkLongtableSpecial(ltt, value, flag);
@@ -5938,6 +5950,7 @@ void InsetTabular::tabularFeatures(Cursor & cur,
 
 	case Tabular::UNSET_LTNEWPAGE:
 		flag = false;
+		// fall through
 	case Tabular::SET_LTNEWPAGE:
 		tabular.setLTNewPage(row, flag);
 		break;

src/output_latex.cpp

@@ -1028,8 +1028,8 @@ void TeXOnePar(Buffer const & buf,
 			break;
 		}
 	}
-
+	// possible
-	// fall through possible
+	// fall through
 	default:
 		// we don't need it for the last paragraph!!!
 		if (nextpar)