MoviePlayer: don't track movies whose parents are also Movies

(This could previously happen when a Movie swaps in a new child Movie on
a keyframe)
Movie.playChildrenOnly

This allows you to stop a movie's main timeline but keep its children
running. This mirrors Flash's "stop()" functionality, where stopping a
Movie doesn't have an effect on its children, and which was previously
not possible with Flump.

This means that:
Movie.play() == MovieClip.play()
Movie.playChildrenOnly() == MovieClip.stop()
Movie.stop() == MovieClip.stopAllMovieClips()
Merge branch 'playChildrenOnly'