MuseScore 07/07/2015: improved Undo action for mixer

I finished my work on basic read/write tests.
I switched to implementing Undo action for mixer, made small improvements in Undo system and fixed a few mistakes in mixer.

My last reports are not very informative because I'm going to finish the implementation of "assigning MIDI channels to instruments" feature. Therefore I mostly fix small bugs and add a lot of small changes.