Update frontend.py

Add definition of handle_play_stop which will allow the ALSA playback device to unlock for audio to be played back from an alternative source

mopidy_raspberry_gpio/frontend.py

@@ -62,6 +62,12 @@ class RaspberryGPIOFrontend(pykka.ThreadingActor, core.CoreListener):
             self.core.playback.pause()
         else:
             self.core.playback.play()
+            
+    def handle_play_stop(self, config):
+        if self.core.playback.get_state().get() == core.PlaybackState.PLAYING:
+            self.core.playback.stop()
+        else:
+            self.core.playback.play()
 
     def handle_next(self, config):
         self.core.playback.next()