player: Add methods and signals needed by mpris

- Make has_next and has_previous public
- Add play_pause method
- Remove mpris functions
- Return playback status as class constants
- Remove variant wrappers on metadata values
- Add playback-status-changed signal
- Add repeat-mode-changed signal
- Add get/set repeat mode methods
- Add volume-changed signal
- Add prev-next-invalidated signal
- Add seeked signal
- Add get/set position methods
mpris: Implement MediaPlayer2.Player interface

Implement all methods, signal and properties, except for Rate property.