Merge branch 'animation-improvements'

Merge branch 'animation-improvements'

* animation-improvements:
  [docs] Add ClutterAnimatable to the API reference
  Add license notice to ClutterAnimation files
  [docs] Update the ClutterAnimation section
  [animation] Extend ClutterAnimation support to all objects
  [animation] Use ClutterAnimatable inside Animation
  [animation] Add ClutterAnimatable
  [animation] Allow registering custom progress function
  [animation] Interval::compute_value should return a boolean
  Animate ClutterColor properties