Better handling of multiple buffers in UndoGroupHelper

Better handling of multiple buffers in UndoGroupHelper

It may happen that the buffers are visited in order buffer1,
buffer2, buffer1. In this case, we want to have only one undo group
in buffer1. The solution is to replace buffer_ with a set<Buffer*>.

A use case among others is InsetLabel::updateReferences.

(cherry picked from commit cc7364dfc50c399e99158fd9efe2f3eb0e6f4570)