MediaIPC: added stop action.

- Be aware that stop can't be resumed with play, from my testing once
stopped the media is no longer available.

Services/IPCService.qml

@@ -215,6 +215,10 @@ Item {
       MediaService.play()
     }
 
+    function stop() {
+      MediaService.stop()
+    }
+
     function pause() {
       MediaService.pause()
     }