AnimComposer.getAnimClips() and .getAnimClipsNames() should return sets, not collections

[stephengold]

Currently in com.jme3.anim (new animation system) the AnimComposer control defines 2 methods to enumerate all clips contained in the composer. While both methods return collections, the implementation does not allow the same AnimClip to be added twice. It would be advantageous to return sets instead, to clarify that duplicates are not allowed.

[rvandoosselaer]

I'll tackle this issues this evening.

[stephengold]

Removed getAnimClips() from the scope of this issue, per discussion at PR #1071.

[stephengold]

The issue is open until #1076 gets integrated.