Rewrite MPRIS implementation using zbus (#1107)

* Rewrite MPRIS implementation using zbus

The initial DBus implementation was getting harder to maintain and `zbus` offers
some nice convenience features that should make our MPRIS implementation
cleaner.

For now this only implements the `org.mpris.MediaPlayer2` interface which does
not do much.

Should help with #1103

* Implement MPRIS properties

- `PlaybackStatus`
- `PlaybackRate`
- `Volume` (get/set)

* Implement remaining player properties/functions

* Emit signal for changed properties on track change

src/spotify_url.rs

@@ -25,7 +25,7 @@ impl fmt::Display for SpotifyUrl {
 }
 
 impl SpotifyUrl {
-    fn new(id: &str, uri_type: UriType) -> SpotifyUrl {
+    pub fn new(id: &str, uri_type: UriType) -> SpotifyUrl {
         SpotifyUrl {
             id: id.to_string(),
             uri_type,