Media controls (MPRIS, Windows SMTC); stop old tracks when switching

Widgets.py

@@ -18,6 +18,8 @@ class App:
         # Set by Main.
         self.SwitchMode: Callable[[str], None] = lambda Name: None
         self.OpenInAudio: Callable[[list[str]], None] = lambda Paths: None
+        self.MediaChanged: Callable[[], None] = lambda: None  # a mode's NowPlaying changed
+        self.MediaModesChanged: Callable[[], None] = lambda: None  # shuffle, repeat or loop changed
 
     def Notify(self, Text: str, Kind: str = "info") -> None:
         Background, Accent = NotificationStyle(Kind)