Tween looping and import fixes

Fix importing clips (vs groups) from the Editor. 

In the Editor, there are two ways of applying end-action for a collection of layers:

- If this collection is just a _folder_ of layers, its band "cuts" the lifetime of its' children and, if some repeat mode is set, resets their time to 0 for every repetition (or just hides them, if not)
- If this collection is a _Group_ (formerly Clip) _and some repeat mode is set_, its band do not cuts the lifetime of its' children (they continue to move and progress without resetting the time, even if the band is finished), but rather cuts the lifetime for its' tweens, so tweens altogether are starting at 0 for every repetition

May be it needs a more beautiful way to implement this. I'll think on this.
can we please not pollute the code with todos and use a tracker or something?