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
UndoGroup helper: 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.