Improve branch activatiion LFUNs

Improve branch activatiion LFUNs

* 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.

Fixes bug #12588.