Add: data parameter in Window::Close method.

This allows passing data when closing a window, e.g. to indicate how it was closed.

src/widgets/dropdown.cpp

@@ -173,7 +173,7 @@ struct DropdownWindow : Window {
 		this->instant_close    = instant_close;
 	}
 
-	void Close() override
+	void Close([[maybe_unused]] int data = 0) override
 	{
 		/* Finish closing the dropdown, so it doesn't affect new window placement.
 		 * Also mark it dirty in case the callback deals with the screen. (e.g. screenshots). */