Stop storing preferences and legacy flags in the saved game

- Nuke global preference variables (they're now fetched with get_xxx_pref whenever needed)
- Nuke magic SDFs that store preferences and other info
- The only preferences now stored in the saved game are those related to difficulty
- play_sound no longer takes an option repeat parameter, but instead takes a delay which will be used if sounds are disabled
- SDF array increased to 350x50
- When saving a legacy scenario, a dialog is shown to remind you to update and to allow you to clear the legacy flag

src/boe.dlgutil.hpp

@@ -23,7 +23,6 @@ void put_party_stats();
 void edit_party();
 void pick_preferences();
 void save_prefs(bool resetHelp = false);
-void load_prefs();
 void tip_of_day();
 short pick_a_scen();
 short pick_prefab_scen();