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

Core.py

@@ -126,6 +126,9 @@ class AppConfig:
     AudioAutoAdvance: bool = False
     # Music mode: keep the queue and position between runs.
     RememberSession: bool = True
+    # Show up in the system's media controls (MPRIS on Linux, the media
+    # overlay on Windows) and answer media keys.
+    MediaControls: bool = True
 
     def Save(self) -> None:
         ConfigDir.mkdir(parents=True, exist_ok=True)