application: Destroy window on quit

The quit method calls Gio.Application's quit method, which is currently not
implemented by Gtk.Application. Destroy the main window for now.
Create new file mpris.py

The file will contain the service for all interfaces of MPRIS implemented
by gnome-music.
application: Create MPRIS service after creating the window
mpris: Implement MediaPlayer2 Interface
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, signals and properties, except for Rate property.
Merge pull request #99 from kyoushuu/mpris

Implement MPRIS