New Session and Mixer mechanism for Failed sources

When a source in a session fails, it is not anymore deleted after update; the Mixer keeps it in the session but detaches it from the scene. This way the user can access the failed source in the navigator (listed in RED), and Replace the source. The Replacement of source is available for any source. The source visitor does not skip a visit if the source failed.

src/UserInterfaceManager.h

@@ -209,6 +209,7 @@ private:
     std::string sourceMediaFileCurrent;
     MediaCreateMode new_media_mode;
     bool new_media_mode_changed;
+    Source *source_to_replace;
 };
 
 class ToolBox