fix transpose and keysig undo

[heuchi]

2 changes in libmscore/undo.cpp:

orig. line 1735:
keysig->staff()->setKey(keysig->segment()->tick(), ks);
was commented out. It does however fix the issue 16506. Do I miss any negative side effects?

starting orig. line 1647:
qSwap(oldElement, newElement) was called too early.
setUpdateKeymap(true) doesn't work because the call to score->cmdUpdateNotes() in Score::undoChangeKeySig will still use the old keymap.

[heuchi]

I decided to combine this and 18105 because they are both about undo/redo. Testing one without the other is almost impossible.
I'm adding a Score::undoChangeTransposingState to create undo events for a new '_transposing' state variable in Score, since updateAccidentals needs to keep all ACC_USER when transposing. When not transposing however, updateAccidentals should reset ACC_USER accidentals that become needed to ACC_AUTO. For redo of a transposition it is neccessary, that _transposing is again set and reset.
I also changed undoChangePitch to know if we're undoing or redoing, since for the latter it has to remove ACC_USER accidentals when the tpc changes.
I hope this makes sense. I'm looking forward to hearing comments.

[heuchi]

This might need some more testing, once the parts function workes properly. Also see http://musescore.org/en/node/18099