Add QMediaBackgroundPlaybackControl interface

Allows some media service backend to provide the ability to can play media in the background while the application is terminated.

Change-Id: Ia5e751367f6d6ea6847c3c873b45df5f971557e2
Reviewed-on: http://codereview.qt.nokia.com/3561
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>

src/multimediakit/qmediaserviceprovider.cpp

@@ -110,6 +110,9 @@ public:
     \value VideoSurface
             The service is capable of renderering to a QAbstractVideoSurface
             output.
+
+    \value BackgroundPlayback
+            The service is capable of doing playback in the background
 */
 
 /*!