player: Make has_next and has_previous public
player: Add play_pause method
mpris: Partially implement MediaPlayer2.Player interface
player: Cleanup duplicated mpris functions

Now that mpris has its own class, we should remove these methods and other
unnecessary things.
player: Return playback status as class constants
mpris: Implement PlaybackStatus
player: Remove variant wrappers on metadata values
mpris: Implement Metadata property
player: Add playback-status-changed signal
mpris: Implement PlaybackStatus property
player: Add repeat-mode-changed signal
player: Add get/set repeat mode methods
mpris: Implement LoopStatus and Shuffle properties
player: Add volume-changed signal
mpris: Implement Volume property
player: Add seeked signal
player: Add get/set position methods
mpris: Implement seeking