Add a test for calling AnimationGroup.add() with an existing child
Remove AnimationGroup.getActiveAnimations()

Remove unused AnimationGroup.getActiveAnimations()

This was added as a stub in 0f337947 and implemented in 31218c11. However, it
was never called, other than by itself.

Animation._getActiveAnimations() and AnimationGroup._getActiveAnimations() were
introduced in 629b270e and these are used by DEFAULT_GROUP.
Add TimedItem._getActiveAnimations()

AnimationGroup.._getActiveAnimations() calls _getActiveAnimations() on its
children, which are of type TimedItem. We should therefore add a stub
implementation of this method to TimedItem.
Add TimedItem.clone()

This is required by the current version of the spec and will be required to
support TimedItem.attach() (part of the TimeSources implementation) in the case
that the TimedItem is already attached.