Fixed initialization of source and of SessionSource; sources are ready

after full initialization and in standly. A new button allows playing
the source (media or session) in the new source pannel.

MediaSource.cpp

@@ -119,7 +119,8 @@ void MediaSource::update(float dt)
     Source::update(dt);
 
     // update video
-    mediaplayer_->update();
+    if (active_)
+        mediaplayer_->update();
 }
 
 void MediaSource::render()