Fix a few bugs and unimplemented features

* Show the now playing cover when playing an item, otherwise show the
  collection cover.
* ItemModelLoaders now correctly expose list properties of non-built-in
  Qt objects
* toString is now implemented for lists, fixing some query
  construction code.
* PlaybackManager now clears the playlist when playing a single item to
  prevent weird behaviour.
* The covers are slightly updated.

core/include/JellyfinQt/model/playlist.h

@@ -100,6 +100,12 @@ public:
      */
     void appendToList(ViewModel::ItemModel &model);
 
+    /**
+     * @brief appendToList Appends a single item to the current list
+     * @param item The item to append
+     */
+    void appendToList(QSharedPointer<Model::Item> item);
+
     /**
      * @brief Start playing this playlist
      * @param index The index to start from.