undo/redo becomes unsafe after load new scenario, so clear it

src/scenedit/scen.actions.cpp

@@ -258,6 +258,8 @@ static bool handle_lb_action(int i){
 				if(!file_to_load.empty() && load_scenario(file_to_load, scenario)) {
 					set_pref("LastScenario", file_to_load.string());
 					restore_editor_state();
+					undo_list.clear();
+					update_edit_menu();
 				} else if(!file_to_load.empty())
 					// If we tried to load but failed, the scenario record is messed up, so boot to start screen.
 					set_up_start_screen();

src/scenedit/scen.appleevents.mm

@@ -52,6 +52,8 @@ void set_up_apple_events() {
 	std::copy(msg.get(), msg.get() + len, std::inserter(fileName, fileName.begin()));
 	
 	if(load_scenario(fileName, scenario)) {
+		undo_list.clear();
+		update_edit_menu();
 		set_pref("LastScenario", fileName);
 		restore_editor_state();
 		change_made = false;

src/scenedit/scen.core.cpp

@@ -3280,6 +3280,9 @@ bool build_scenario() {
 	if(!edit_make_scen_2(width, height, lg, med, sm, default_town))
 		return false;
 	
+	undo_list.clear();
+	update_edit_menu();
+
 	scenario = cScenario();
 	scenario.scen_name = title;
 	scenario.contact_info[0] = author;

src/scenedit/scen.main.cpp

@@ -332,6 +332,8 @@ static void process_args(int argc, char* argv[]) {
 	}
 	if(!file.empty()) {
 		if(load_scenario(file, scenario)) {
+			// On first launch, no need to clear the undo_list
+
 			set_pref("LastScenario", file);
 			restore_editor_state();
 			change_made = false;
@@ -516,6 +518,8 @@ void handle_menu_choice(eMenu item_hit) {
 				break;
 			file_to_load = item_hit == eMenu::FILE_OPEN ? nav_get_scenario() : scenario.scen_file;
 			if(!file_to_load.empty() && load_scenario(file_to_load, scenario)) {
+				undo_list.clear();
+				update_edit_menu();
 				set_pref("LastScenario", file_to_load.string());
 				restore_editor_state();
 				change_made = false;