Implemented the following getters for MPRIS:
- CanQuit
- CanRaise
- HasTrackList
- Identity
- DesktopEntry
- SupportedUriSchemas
- SupportedMimeTypes
- Metadata
- CanPlay / CanPause / CanSeek
- CanControl
- PlaybackStatus
Implemented a koto_push_track_info_to_builder function that enables us to easily push KotoIndexedTrack as well as associated album and artist info to a GVariantBuilder for use in a GVariant for various getters.
Implemented a koto_update_mpris_info_for_track function that emits a signal for PropertiesChanged + "Metadata" when our track info changes.
