Mpris2: Add new property to read/write the rating

[Dakes]

This is to create shortcuts to set the rating and/or to set it from scripts.
This feature was wished for in these threads:
https://forum.strawberrymusicplayer.org/topic/99/shortcuts-for-ratings
https://forum.strawberrymusicplayer.org/topic/326/set-ratings-per-dbus-shortcut
https://forum.strawberrymusicplayer.org/topic/879/expose-rating-of-songs-to-automation-shortcuts

This is technically out of spec for the mpris specification. But since it does not modify any of the original methods it should be fine. If you prefer I could move this to a different bus name like org.strawberry.Player.Rating or something.

Currently it can for example be accessed like this: qdbus org.mpris.MediaPlayer2.strawberry /org/mpris/MediaPlayer2 org.mpris.MediaPlayer2.Player.Rating to get the rating of the currently playing song (As float from 0-1)
Or qdbus org.mpris.MediaPlayer2.strawberry /org/mpris/MediaPlayer2 org.mpris.MediaPlayer2.Player.Rating 0.6 to set the rating to a float, here 0.6. (from 0-1)

[jonaski]

Thanks