This requires adding another notion of mouse-release events to `ClipEvent`. We now have four:
* `MouseUp` - the mouse was released, any object on the render list can handle this event ("anycast" event)
* `MouseUpInside` - the mouse was released inside this display object, only the mouse-picked target of the event can handle it
* `Release` - the mouse was released inside the last clicked display object
* `ReleaseOutside` - the mouse was released outside the last clicked display object
For those keeping score at home, in AVM2, the valid progression of events is either...
* On the same object, `mouseDown`, `mouseUp`, and `click`
* On one object, `mouseDown`, then some mouse movement that takes the cursor out of the first object, then on another object `mouseUp`, and then finally the first object gets `releaseOutside`.
This makes `DesktopUiBackend::is_key_down` a simple one-line check.
Also unify the handling code of `ElementState::Pressed` and `ElementState::Released`,
which had a lot of common code.
* 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.
Aaron Hill