Use the proper buffer when recording undo

Use the proper buffer when recording undo

The code in InsetLabel::updateReferences code changes reference insets
in potentially several buffers. When recording undo, it is important
to use the right undo stack, otherwise crashes can ensue.

Once it is done, it is neccessary to create undo groups as needed.
This is done using UndoGroupHelper. This demonstrates a shortcoming of
UndoGroupHelper: if a buffer is encountered in two seperate
occasions, two undo groups will be created for this buffer. This is
not correct.

Fixes bug #10643.

(cherry picked from commit 4eb9b50dc642a3532d8126a93571696b3db57640)