New Source Callbacks for Play control )fast forward, seek, etc.)

2 new callbacks (PlayFastForward, PlaySpeed), modified Seek callback to take target time in seconds (instead of ratio of duration). Integrating this in Input Mapping GUI and Session saving.

src/Mixer.cpp

@@ -214,7 +214,6 @@ void Mixer::update()
             // recreating it in the current session
             RenderSource *failedRender = dynamic_cast<RenderSource *>(*it);
             if (failedRender != nullptr) {
-                g_printerr("%s failed render \n", failedRender->initials());
                 // try to recreate the failed render source
                 if ( !recreateSource(failedRender) )
                     // delete the source if could not