minor Doxyfile changes: disable Latex output
gtk-vlc-player widget: fixed VLC callback processing (dead locks)

either libVLC 2.0.5 changed the semantics of their callbacks (they can be
invoked from the main thread, i.e. from the VLC method resulting in the event);
or GTK+ 2.24 silently changed the type of the GDK mutex from recursive to simple.

in either case to avoid GDK mutex deadlocks we must check whether the VLC
callback's thread already holds the lock or owns the main context