Allow exit panel to be bound to a key shortcut

While debugging the save/restore workspace state, I found quite
useful to be able to exit windowmaker using a key shortcut.

WindowMaker/Defaults/WindowMaker.in

@@ -232,6 +232,7 @@
 	WindowRelaunchKey = None;
 	ScreenSwitchKey = None;
 	RunKey = None;
+	ExitKey = None;
 	NormalCursor = (builtin, left_ptr);
 	ArrowCursor = (builtin, top_left_arrow);
 	MoveCursor = (builtin, fleur);