Refactor of Tween classes (calling start() is mandatory) - to address is...

...sue #128:

 - all tween packages/classes are affected by these changes (misc, motion, sound, FP)
 - the call to start() has been removed from tween()
 - tween() returns the (specific) tween itself for chaining
 - Tween.start() also returns the tween itself for chaining (you'd probably have to cast it to the proper tween)

Why? `addTween(_someTween_, true/false)` should now behave correctly.


I've tested it... maybe not thoroughly but... Any thoughts/comments about this?