Tweak New Scenario dialog and add Save As option

- You now specify your name instead of the scenario filename in the first dialog
- A file dialog pops up after the second dialog, to let you choose the filename and location
- Fixed using Warrior's Grove even if you chose not to
- Fixed not recognizing .EXS as a valid legacy-scenario extension and appending .boes, resulting in .EXS.boes
- Adjust Difficulty scenario flag set on by default

src/scenedit/scen.actions.cpp

@@ -2802,7 +2802,7 @@ bool save_check(std::string which_dlog) {
 	else if(choice == "cancel")
 		return false;
 	town->set_up_lights();
-	save_scenario(scenario.scen_file);
+	save_scenario();
 	return true;
 }