Replace Tweener with Clutter's implicit animations for animating actors

This is still work-in-progress, but keeping it lying around on my local system won't make it any good ...

This is good for review now (although !654 (merged) whose commits are included here should go first), so resolving WIP status.

The patch set now no longer hooks into Tweener to transparently use Clutter animations, instead it opts for an explicit port. It does this in three parts:

1. adjust some tweens that use patterns that prevent replacing them with Clutter animations (mostly using a proxy JS property )
2. add some convenience API (not unlike the Tweener API)
3. replace all tweens of animatable properties with implicit animations

That last step reduces Tweener usage by over 80%; the remaining bits are either animating properties on custom actor subclasses or non-actors (layout/effect properties, adjustment values, ...). All of those will be replaced in follow-up merge requests.