Changed KeyframeFormat and LayerFormat to use the @:optional metadata rather than using :Null<Type>
framelabel support changes/updates
- Allow frame-labels on any layer
- Moved most of the label/layer support into MovieSymbol/MovieLayer,
- Now each LayerAnimator in a MovieSprite manages a currentLabel and labelChanged property in the .composeFrame call.
specify optional values with ?(for nullability)