MediaPlayer: reintroduce "stop" shortcut.

It turns out there are some people actually using it, mainly when
playing music. If you interrupt music, you often want to start listeing
from the start of the track again (unlike for video), so having a stop
button makes sense in that case.

src/apps/mediaplayer/MainWin.cpp

@@ -2257,8 +2257,9 @@ MainWin::_KeyDown(BMessage* msg)
 		case 0x4e:
 			fController->Pause();
 			return true;
-		// 0x4f would be the "stop" button, but this is too dangerous to be
+		case 0x4f:
-		// triggered with a single key (it loses the current position)
+			fController->Stop();
+			return true;
 		case 0x50:
 			PostMessage(M_SKIP_NEXT);
 			return true;