Rename Timeline.Frame to Timeline.FrameDelta

... which actually describes what the class does.
Store start offset in Scene objects
Cleanups in Scene class
Move all scene- and label-related calculations out of MovieClip's hot path

These are only used some of the time, so making them a tiny bit more expensive isn't a problem. `_advanceFrame`, OTOH, happens all the time, and should do as little work as possible.

Additionally, this saves 4 fields on MovieClip - not too shabby, I think.

And it's even less code than what was there previously.
Merge pull request #1398 from tschneidereit/nat

MovieClip optimizations