Strong typedef: Use strong typedefs for date, date tick, minutes types

Add delta types
Adjust/add type conversion functions
Add various utility methods on types
Remove the various minute macros
Fix some minute conversion inconsistencies

src/sl/misc_sl.cpp

@@ -162,7 +162,7 @@ static void Check_DATE()
 {
 	SlGlobList(_date_check_desc);
 	if (IsSavegameVersionBefore(SLV_31)) {
-		_load_check_data.current_date += DAYS_TILL_ORIGINAL_BASE_YEAR;
+		_load_check_data.current_date += DAYS_TILL_ORIGINAL_BASE_YEAR.AsDelta();
 	}
 }