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/spotify.rs

@@ -415,6 +415,8 @@ impl Spotify {
     /// Seek in the currently played [Playable] played by the [Player].
     pub fn seek(&self, position_ms: u32) {
         self.send_worker(WorkerCommand::Seek(position_ms));
+        #[cfg(feature = "mpris")]
+        self.notify_seeked(position_ms);
     }
 
     /// Seek relatively to the current playback position of the [Player].
@@ -429,6 +431,19 @@ impl Spotify {
         self.cfg.state().volume
     }
 
+    /// Send a Seeked signal on Mpris interface
+    #[cfg(feature = "mpris")]
+    pub fn notify_seeked(&self, position_ms: u32) {
+        let new_position = Duration::from_millis(position_ms.into());
+        let command = MprisCommand::EmitSeekedStatus(
+            new_position
+                .as_micros()
+                .try_into()
+                .expect("track position exceeds MPRIS datatype"),
+        );
+        self.send_mpris(command);
+    }
+
     /// Set the current volume of the [Player]. If `notify` is true, also notify MPRIS clients about
     /// the update.
     pub fn set_volume(&self, volume: u16, notify: bool) {