Recording single changes of objects or their specific properties is
completely superfluous as we have full implemented state tracking in all
objects already. Therefore use SerializingObject::saveState() and
SerializingObject::restoreState() in order to implement the undo/redo
functionality.
This is just an initial commit and needs some further work (especially
regarding stability). However even things like undo/redo of
addition/removal of Tracks and TrackContentObjects do work already.
a5c3cf6a99