playlist/Queue: add one-shot to single mode

src/player/Listener.hxx

@@ -31,6 +31,11 @@ public:
 	 * The current song's tag has changed.
 	 */
 	virtual void OnPlayerTagModified() noexcept = 0;
+
+	/**
+	 * Playback went into border pause.
+	 */
+	virtual void OnBorderPause() noexcept = 0;
 };
 
 #endif

src/player/Thread.cxx

@@ -930,6 +930,7 @@ Player::SongBorder() noexcept
 	const bool border_pause = pc.ApplyBorderPause();
 	if (border_pause) {
 		paused = true;
+		pc.listener.OnBorderPause();
 		idle_add(IDLE_PLAYER);
 	}
 }