Nightly 2021-06-12

core: Use swf::ClipEventFlag in stored clip event handlers

 * Rename movie_clip::ClipAction to movie_clip::ClipEventHandler.
 * Store the swf::ClipEventFlag event flags that trigger the event
   directly in the event handler. Previously we split up any event
   that had multiple event flags into separate events. Now these
   can be kept as a single event.
 * Remove `MovieClip::has_button_event`, and instead store the
   union of all event flags in `MovieClip::clip_event_flags`. This
   will be useful for other cases in the future.