Song: cleanup and coding style improvements
Song: revert accidental change
BbTrack: replaced static numOfBBTrack() by proper index() method
BbEditor: use foreach() loop
Merge remote-tracking branch 'origin/stable-1.0'
TrackContentObjectView: add journal checkpoint in remove() rather than close()

The remove() slot is only called upon user interaction (i.e. right click)
and thus the correct place where to make checkpoints.
TrackContainerView: add journal checkpoints in view rather than model
TrackContainerView: disable undo/redo for track creation/removal

Due to the complex implementation of BB tracks it's hard to implement
a reliable undo/redo journalling for whole TrackContainerViews.

Closes #531.