TimelineObserver._clipPropertyChangedCb, the final handler of the prop
changed events ("monitored-property-changed"), which pushes the
ClipPropertyChanged actions, was ignoring these events when the clips
were being dragged around. To be able to record the final changes,
ClipPropertyChangeTracker had to connect to the timeline's "commited"
signal and report if changes have been detected.

The goal was to have as few as possible actions in the UndoableActionLog
because when undoing or redoing, all these actions were committing the
timeline and this was very expensive. Now the individual actions are not
committing the timeline anymore, so we can live with a high number of
actions in the stack.

As a result of the simplification, the TimelineObserver does not need to
keep track of the prop values itself (by setting attributes on the
ClipPropertyChangeTracker) and the "monitored-property-changed" signal
can be removed as now PropertyChangeTracker can push the
PropertyChangedAction instances itself.

Finally, PropertyChangeTracker can now be used easily to monitor the
props of other objects besides clips, stay tuned!

Differential Revision: https://phabricator.freedesktop.org/D993