new car.quit callback for user scripts

This is particularly important for scripts that use my editor widget and
need to invoke edit.quit. But it also seems like a good idea anyway.

0106-stop_button

@@ -1,9 +1,12 @@
 stop_button = function(x, y)
 	styled_button('stop', x,y,
 		function()
+			if car.quit then
+				call_protected(car.quit)
+			end
 			Show_menu = nil
 			clear_handlers()
 		end)
 	local w = App.width('stop')+10
 	return x+w+10, y
-end
+end