TV: Clear the contents of TvPlayWindow when playback is starting.

This ensures we don't see the 'Please wait' etc popups when exiting
playback and clears up some theme anomalies with the TvPlayWindow
appearing behind, for example, the program guide in Graphite.
(cherry picked from commit 6aef308)

mythtv/libs/libmythtv/tv_play.cpp

@@ -2252,6 +2252,9 @@ void TV::HandleStateChange(PlayerContext *mctx, PlayerContext *ctx)
             GetMythMainWindow()->PushDrawDisabled();
         }
         DrawUnusedRects();
+        // we no longer need the contents of myWindow
+        if (myWindow)
+            myWindow->DeleteAllChildren();
     }
 
     VERBOSE(VB_PLAYBACK, LOC +