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.fileio.hpp

@@ -1,5 +1,5 @@
 
-void save_scenario(fs::path toFile);
+void save_scenario(bool rename = false);
 
 void start_data_dump();
 void scen_text_dump();