I don't really like the restriction of requiring a `labels` layer. Shouldn't it be possible to search for labels across all layers? If you have multiple different frames with the same label, the behavior could be undefined.

The label searching and mapping should happen in the MovieSymbol/MovieLayer/MovieKeyframe data classes at Library load-time. Since you can have many MovieSprites that point at the same symbol data, that would save a lot of memory/CPU.