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()
     }

Services/MediaService.qml

@@ -200,6 +200,13 @@ Singleton {
     }
   }
 
+  function stop() {
+    let target = currentPlayer ? (currentPlayer._controlTarget || currentPlayer) : null
+    if (target) {
+      target.stop()
+    }
+  }
+
   function pause() {
     let target = currentPlayer ? (currentPlayer._controlTarget || currentPlayer) : null
     if (target && target.canPause) {