Codechange: Optionally allow passing state to GUIList sorter function.

GUIList sorter functions can currently only use global state, which makes per-window-instance sorting difficult.

src/signs_gui.cpp

@@ -41,7 +41,7 @@ struct SignList {
 	/**
 	 * A GUIList contains signs and uses a StringFilter for filtering.
 	 */
-	typedef GUIList<const Sign *, StringFilter &> GUISignList;
+	typedef GUIList<const Sign *, std::nullptr_t, StringFilter &> GUISignList;
 
 	GUISignList signs;