Fix bug #8242: undo fails for longest label width

Fix bug #8242: undo fails for longest label width

In the existing code for setting layout of pragraph parameters,
Text::undoSpan is used to "guess" which paragraphs should be saved in Undo.
With this patch the approach is more precise: before every explicit change
to a paragraph, a Cursor::recordUndo call is inserted. This is much more robust than trying to guess.

In particular, we do not look at depth changes at all, since they are now
handled in updateBuffer since #8159 has been fixed.