Play children only

@aduros, @groves, @roguenet: this branch implements `Movie.playChildrenOnly`, which basically mirrors the behavior of Flash's `Movie.stop` - that is, stop the movie's timeline but not those of its children. This is something I need for my game.

This implements the feature that @roguenet requested in https://github.com/threerings/flump/issues/67, but does it in a non-breaking way. The interface remains the same.

Any objections? I can see possible objections to the name?