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()
Movie +stopAt, +play
child movie not advanceTimed on same frame as it is added to layer
remove errant newline
formatting fix
Merge pull request #96 from kpatelPro/child-movie-advance-time-fix

Child movie advance time fix
Merge pull request #94 from kpatelPro/movie-set-stop-position

Add stopAt(position), play() to Movie interface
Movie.addedToLayer is internal
Remove superfluous conditional

If _stopFrame is NO_FRAME (-1) _frame should never equal it
Merge branch 'playChildrenOnly'
update to AIR 3.9
layers use visibility rather than add/remove Child when switching displays; cache numFrames; remove clamp method