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: Fix playback status
player: Remove variant wrappers on metadata values
mpris: Wrap metadata in a dictionary
mpris: Connect to signal current-changed of player

Update the metadata when the player emits current-changed.
player: Add new signal playback-status-changed to Player
mpris: Connect to signal playback-status-changed of Player

Update the playback status when the player status changes.