Try to end the neverending fixIfBroken saga

This is a fixup to the  series of commits 522516d9, d0acc3e5, 13c3c148.

Actually the right thing to consider in all situations is isActive(),
which really means `can a cursor be in there?'.

Improve the description of isActive() and editable() in Inset.h.

Set isActive() to false for InsetInfo, since no cursor should ever go there.

Again part of #10667.

src/DocIterator.cpp

@@ -562,9 +562,7 @@ bool DocIterator::fixIfBroken()
 	size_t n = slices_.size();
 	for (; i != n; ++i) {
 		CursorSlice & cs = slices_[i];
-		if (&cs.inset() != inset
-		    || (cs.inset().inMathed() ? ! cs.inset().isActive()
-		                              : ! cs.inset().editable())) {
+		if (&cs.inset() != inset || ! cs.inset().isActive()) {
 			// the whole slice is wrong, chop off this as well
 			--i;
 			LYXERR(Debug::DEBUG, "fixIfBroken(): inset changed");

src/insets/Inset.h

@@ -342,18 +342,18 @@ public:
 							 size_t const maxlen = TOC_ENTRY_LENGTH,
 							 bool const shorten = true) const;
 
+	/// Can a cursor be put in there ?
+	/// Forced to false for insets that have hidden contents, like
+	/// InsetMathCommand and InsetInfo.
+	virtual bool isActive() const { return nargs() > 0; }
 	/// can the contents of the inset be edited on screen ?
-	// true for InsetCollapsables (not ButtonOnly) (not InsetInfo), InsetText
+	// equivalent to isActive except for closed InsetCollapsable
 	virtual bool editable() const;
 	/// has the Inset settings that can be modified in a dialog ?
 	virtual bool hasSettings() const;
 	/// can we go further down on mouse click?
-	// true for InsetCaption, InsetCollapsables (not ButtonOnly), InsetTabular
+	/// true for InsetCaption, InsetCollapsables (not ButtonOnly), InsetTabular
 	virtual bool descendable(BufferView const &) const { return false; }
-	/// is this an inset that can be moved into?
-	/// FIXME: merge with editable()
-	// true for InsetTabular & InsetText
-	virtual bool isActive() const { return nargs() > 0; }
 	/// can we click at the specified position ?
 	virtual bool clickable(BufferView const &, int, int) const { return false; }
 	/// Move one cell backwards

src/insets/InsetInfo.h

@@ -101,6 +101,8 @@ public:
 	///
 	Inset * editXY(Cursor & cur, int x, int y);
 	///
+	bool isActive() const { return false; }
+	///
 	bool editable() const { return false; }
 	///
 	bool hasSettings() const { return true; }