mpris: Use player Identity if the player app isn't known

For players that don't match to an app, we can still do better than
the generic fallback if the client provides a valid `Identity` value.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3103>

data/dbus-interfaces/org.mpris.MediaPlayer2.xml

@@ -3,5 +3,6 @@
     <method name="Raise"/>
     <property name="CanRaise" type="b" access="read"/>
     <property name="DesktopEntry" type="s" access="read"/>
+    <property name="Identity" type="s" access="read"/>
   </interface>
 </node>

js/ui/mpris.js

@@ -227,7 +227,7 @@ export class MprisPlayer extends Signals.EventEmitter {
         }
 
         this.source.set({
-            title: this._app?.get_name() ?? null,
+            title: this._app?.get_name() ?? this._mprisProxy.Identity,
             icon: this._app?.get_icon() ?? null,
         });