groovectl: remove interactive mode

It's broken and kinda useless right now. I'll redo it (maybe).
dbus: move dbus xml to its own file
player: mpv doesn't allow no-foo=yes anymore
player: add support for single-track looping

This basically reworks the whole Loop dbus method, which now takes a mode string
parameter which can be either "track", "list" or "none", instead of a bool.