Fix to #8159: Undo doesn't restore environment depth correctly

Fix to #8159: Undo doesn't restore environment depth correctly

The idea is to record undo at the place where the document is modified:

1/ in Buffer::updateBuffer, add a recordUndo, with the caveat that a
   const_cast has to be used (because updateBuffer is const but
   modifies the document, go figure).

2/ in GuiApplication::dispatch, add an extra undo group that
   encompasses the updateBuffer call. Some other undo groups may be
   redundant now, but it is not a problem since they do not cost
   anything.