Convert some windows to QDockWidget

I propose to convert some windows to QDockWidgets because they are move flexible and it's more convenient to have single window interface by default.

I propose to convert:
1. Master Palette
2. Play panel
3. Navigator
4. Mixer

I think that Master Palette can be merged into Palettes. The only problem is that 2 widgets are too big to put them into QDockWidget: Create Key Signature and Create Time Signature. I propose make them top-level widgets (QDialog I think is the best choice) and execute them from appropriate places (see mockup).