Expose availability from the backend to C++ and QML.

The availabilityError property was static based on the service, but it
can change at run time, so add the plumbing to allow the backend to
report it itself.

Also make sure that both QML and C++ expose the availability.

The radio tuner and data controls previously had properties (but no
signals) for availability - these have been removed.

Change-Id: I9240cf93e2a51b14cd38642f9312ae3c75f05361
Reviewed-by: Ling Hu <ling.hu@nokia.com>

src/imports/multimedia/Video.qml

@@ -192,6 +192,28 @@ Item {
     */
     property alias errorString:     player.errorString
 
+    /*!
+        \qmlproperty enumeration Video::availability
+
+        Returns the availability state of the video element.
+
+        This is one of:
+        \table
+        \header \li Value \li Description
+        \row \li MediaPlayer.Available
+            \li The video player is available to use.
+        \row \li MediaPlayer.Busy
+            \li The video player is usually available, but some other
+               process is utilizing the hardware necessary to play media.
+        \row \li MediaPlayer.Unavailable
+            \li There are no supported video playback facilities.
+        \row \li MediaPlayer.ResourceMissing
+            \li There is one or more resources missing, so the video player cannot
+               be used.  It may be possible to try again at a later time.
+        \endtable
+     */
+    property alias availability:    player.availability
+
     /*!
         \qmlproperty bool Video::hasAudio