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.menus.hpp

@@ -14,7 +14,7 @@ void shut_down_menus(short mode);
 
 enum class eMenu {
 	NONE, ABOUT, QUIT, FRILL, UNFRILL,
-	FILE_NEW, FILE_OPEN, FILE_CLOSE, FILE_SAVE, FILE_REVERT,
+	FILE_NEW, FILE_OPEN, FILE_CLOSE, FILE_SAVE, FILE_SAVE_AS, FILE_REVERT,
 	EDIT_UNDO, EDIT_REDO, EDIT_CUT, EDIT_COPY, EDIT_PASTE, EDIT_DELETE, EDIT_SELECT_ALL,
 	HELP_TOC, HELP_START, HELP_TEST, HELP_DIST, HELP_CONTEST,
 	// Scenario menu