Implement a working MPRIS DBus connection with a separate API to control it. Right now only player playback state is reflected and Play/Pause/PlayPause calls work.
The customized GTK4 sink version has few differences from the one shipped as part of GStreamer. Rename custom sink to GstClapperGLSink to avoid confusion.
Ship custom gtk4glsink plugin as part of API insead of normal gstreamer plugin. This avoids gstreamer plugin registry conflicts with gtk3 plugin and allows more customization.