Change the way a playlist is bound to a media object.

The previous behavior was to simply switch from the internal
control to the service's control, discarding anything that was
added to the playlist before binding.
We now carry over the changes made to the playlist when switching
controls. This means the switch is now transparent to the user.
When the service's control is read-only, we cannot transfer the
items, which means the user must be notified of the items that
might have been "lost" during the switch.

Auto-test modified to reflect this change.

Change-Id: Ibf80b650b06425ddbaeb320b72ac5d3082a25960
Reviewed-by: Jim Hodapp <jim.hodapp@canonical.com>
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>

tests/auto/unit/qmultimedia_common/mockplaylistservice.h

@@ -44,7 +44,7 @@ class MockPlaylistService : public QMediaService
 public:
     MockPlaylistService():QMediaService(0)
     {
-        mockControl = new MockMediaPlaylistControl(this);
+        mockControl = new MockMediaPlaylistControl(false, this);
     }
 
     ~MockPlaylistService()