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.