feat(mpris): Emit Seeked signal

* Add Seeked signal to Mpris interface

The Mpris2 spec includes a `Seeked` signal which should be fired when
the track position changes in an unexpected way i.e. when the user
seeks to a different part of the track.

This PR implements this signal on seek events and also when a new track
begins. The latter is not strictly required but has been observed in
other players (e.g. VLC).

Closes #1492

* chore: Use `send_mpris()` and `Duration` for conversion

* doc: Update CHANGELOG

---------

Co-authored-by: Henrik Friedrichsen <henrik@affekt.org>

src/queue.rs

@@ -313,6 +313,10 @@ impl Queue {
                     move || send_notification(&summary_txt, &body_txt, cover_url)
                 });
             }
+
+            // Send a Seeked signal at start of new track
+            #[cfg(feature = "mpris")]
+            self.spotify.notify_seeked(0);
         }
 
         if reshuffle && self.get_shuffle() {