switch from (lib-)xine to (lib-)vlc

- many issues (esp. crashes) vanish / are easier to fix
- likely you need the 'full' vlc package (with X11 plugins)
- this commit introduces introduces various regressions ;-)
- big picture problems with libvlc:
- dvd navigation (likely solved with libvlc 1.2 once it's released)
- VDPAU
- make MediaWidget more state-orientated, i.e. separate between
- functions, which modify the state (e.g. a signal from the backend)
- functions, which rely on the state (e.g. ui updates)