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>

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Emit MPRIS `Seeked` signal
+
 ### Fixed
 
 - Switch to OAuth2 login mechanism