[media.vlc] Prevent deadlock on media.vlc.quit.

`_on_stop_event` may be set by the callback, but then cleared again when
`_reset_state` is called.

This can result in the `_on_stop_event.wait` call in `quit` to time out.

Instead, `_on_stop_event` should be cleared only when the player goes
into `playing` or `paused` mode. It's only then that we know for sure
that the state isn't `stopped`, and only in that case it makes sense to
wait for a stop.

platypush/plugins/media/vlc/__init__.py

@@ -126,7 +126,6 @@ class MediaVlcPlugin(MediaPlugin):
 
     def _reset_state(self):
         self._latest_seek = None
-        self._on_stop_event.clear()
 
         if self._latest_resource:
             self.logger.debug('Closing latest resource')
@@ -175,8 +174,10 @@ class MediaVlcPlugin(MediaPlugin):
 
             self.logger.debug('Received VLC event: %s', event.type)
             if event.type == EventType.MediaPlayerPlaying:  # type: ignore
+                self._on_stop_event.clear()
                 self._post_event(MediaPlayEvent, resource=self._get_current_resource())
             elif event.type == EventType.MediaPlayerPaused:  # type: ignore
+                self._on_stop_event.clear()
                 self._post_event(MediaPauseEvent)
             elif event.type in (
                 EventType.MediaPlayerStopped,  # type: ignore