undo/redo for town variable entry

src/scenedit/scen.undo.cpp

@@ -898,4 +898,14 @@ bool aEditAdvancedDetails::undo_me() {
 bool aEditAdvancedDetails::redo_me() {
 	scen_set_advanced(scenario, new_advanced);
 	return true;
+}
+
+bool aEditTownVarying::undo_me() {
+	scenario.town_mods = old_varying;
+	return true;
+}
+
+bool aEditTownVarying::redo_me() {
+	scenario.town_mods = new_varying;
+	return true;
 }