sushi-media-bin: Remove usage of gtkglsink

GtkGstGLWidget does not support reparenting to a different toplevel, and a temporary workaround was applied to reconstruct the whole pipeline when switching to fullscreen mode. This hackish workaround is broken with recent versions of GStreamer, as we never get the event of the availability of the reconstructed pipeline, causing a freeze of the whole app as described in #103. Since reparenting still not supported, and the temporary workaround is broken, the best what we can do is to remove the usage of gtkglsink, and use gtksink instead. gtksink supports reparenting, so the logic can be simplified a lot, and we don't even need to check the availability of the GL context.