Set _current in init instead of ctor.

This allows an animation to be re-initialized and reused.
Various AnimGroup bug fixes.

1. We have to init our sub-animations, duh.

2. We can be repeated, so we have to keep a canonical copy of our
sub-animations and copy those into a "current" array, which can be nulled out
as animations complete.

3. We need to call apply(Animator,float) on our sub-animations, not
apply(float), otherwise chains of animations are not processed properly.
Added a demo of anim groups.