Second tentative fix to #8159: Undo doesn't restore environment depth correctly
In this version, the idea is to record undo at the place where the document is modified, which is definitely cleaner.
1/ in Buffer::updateBuffer, add a recordUndo, with the caveat that a
const_cast has to be used
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.