* put the code that is called both from Buffer and InsetBrach in the
two helper methods Buffer::branchActivationStatus() and
Buffer::branchActivationDispatch().
* Cleanup the code so that _MASTER_ lfuns are disabled when there is
no master document.
* When changing branches in the master buffer, make the buffer visible
if it is not, and make sure that undo information is recorded.
* The code in Buffer::dispatch is used first, and it gives control to
the branch inset code if no branch name has been specified.