undo: Rename effect related tracker to TrackElement

Because we can track children properties of any TrackElement in there.
undo: Remove the effect.py file and move its content to timeline.py

It is not effect specific anymore, be coherent.
titleeditor: Implement Undo Redo

Keeping things as simple as possible using the
GES.TrackElement.s[g]et_child_property API everywhere.