Make inset-select-all select all cells only in tables

inset-select-all has 3 levels
1. select current cell
2. select all cells
3. select inset from outside.

The second level makes sense for tables (text and math), but not for things like a math fraction.

Introduce a new method Inset::isTable() that allows to detect this case properly and skip level 2.

src/BufferView.cpp

@@ -1777,21 +1777,25 @@ void BufferView::dispatch(FuncRequest const & cmd, DispatchResult & dr)
 	}
 
 
-	case LFUN_INSET_SELECT_ALL:
-		if (cur.depth() > 1
+	case LFUN_INSET_SELECT_ALL: {
+		// true if all cells are selected
+		bool const all_selected = cur.depth() > 1
 		    && cur.selBegin().at_begin()
-		    && cur.selEnd().at_end()) {
-			// All the contents of the inset if selected.
+		    && cur.selEnd().at_end();
+		// true if some cells are selected
+		bool const cells_selected = cur.depth() > 1
+		    && cur.selBegin().at_cell_begin()
+			&& cur.selEnd().at_cell_end();
+		if (all_selected || (cells_selected && !cur.inset().isTable())) {
+			// All the contents of the inset if selected, or only at
+			// least one cell but inset is not a table.
 			// Select the inset from outside.
 			cur.pop();
 			cur.resetAnchor();
 			cur.setSelection(true);
 			cur.posForward();
-		} else if (cur.selBegin().idx() != cur.selEnd().idx()
-			   || (cur.depth() > 1
-				   && cur.selBegin().at_cell_begin()
-			       && cur.selEnd().at_cell_end())) {
-			// At least one complete cell is selected.
+		} else if (cells_selected) {
+			// At least one complete cell is selected and inset is a table.
 			// Select all cells
 			cur.idx() = 0;
 			cur.pos() = 0;
@@ -1811,6 +1815,7 @@ void BufferView::dispatch(FuncRequest const & cmd, DispatchResult & dr)
 		cur.setCurrentFont();
 		dr.screenUpdate(Update::Force);
 		break;
+	}
 
 
 	// This would be in Buffer class if only Cursor did not

src/insets/Inset.h

@@ -347,6 +347,8 @@ public:
 	virtual bool clickable(BufferView const &, int, int) const { return false; }
 	/// Move one cell backwards
 	virtual bool allowsCaptionVariation(std::string const &) const { return false; }
+	// true for insets that have a table structure (InsetMathGrid, InsetTabular)
+	virtual bool isTable() const { return false; }
 
 	/// does this contain text that can be change track marked in DVI?
 	virtual bool canTrackChanges() const { return false; }

src/insets/InsetTabular.h

@@ -881,6 +881,8 @@ public:
 	bool inheritFont() const { return false; }
 	///
 	bool allowsCaptionVariation(std::string const &) const;
+	//
+	bool isTable() const { return true; }
 	///
 	DisplayType display() const;
 	///

src/mathed/InsetMathGrid.h

@@ -154,6 +154,8 @@ public:
 	InsetMathGrid * asGridInset() { return this; }
 	/// identifies GridInset
 	InsetMathGrid const * asGridInset() const { return this; }
+	//
+	bool isTable() const { return true; }
 	///
 	col_type ncols() const;
 	///