Changing day length or date no longer changes time in minutes

Add offset variables for scaled date ticks.
Adjust offset when changing day length or date, such that scaled
date ticks remain the same.
Store _scaled_tick_counter and scaled date ticks offset in the savagame.

src/statusbar_gui.cpp

@@ -115,7 +115,7 @@ struct StatusBarWindow : Window {
 		Dimension d;
 		switch (widget) {
 			case WID_S_LEFT:
-				SetDParam(0, (uint64)MAX_YEAR * (uint64)DAYS_IN_YEAR * (uint64)DAY_TICKS * (uint64)_settings_game.economy.day_length_factor);
+				SetDParam(0, DateToScaledDateTicks(MAX_YEAR * DAYS_IN_YEAR));
 				d = GetStringBoundingBox(STR_WHITE_DATE_WALLCLOCK_LONG);
 				break;