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.

sailfish/qml/pages/itemdetails/BaseDetailPage.qml

@@ -103,7 +103,7 @@ Page {
         if (status === PageStatus.Active) {
             console.log("Page ready, ItemID: ", itemId, ", UserID: ", apiClient.userId)
             jItemLoader.autoReload = true
-            //appWindow.itemData = jItemLoader.data
+            appWindow.itemData = Qt.binding(function() { return jItemLoader.data; })
         }
     }
 }