Add very and extremely slow options to town growth rate setting

src/table/settings.ini

@@ -61,6 +61,8 @@ static bool CheckSharingAir(int32 p1);
 
 /* Begin - GUI order callbacks */
 
+static int OrderTownGrowthRate(uint nth);
+
 /* End - GUI order callbacks */
 
 /* Some settings do not need to be synchronised when playing in multiplayer.
@@ -1738,15 +1740,16 @@ extver   = SlXvFeatureTest(XSLFTO_AND, XSLFI_SPRINGPP)
 [SDT_VAR]
 base     = GameSettings
 var      = economy.town_growth_rate
-type     = SLE_UINT8
+type     = SLE_INT8
 from     = 54
 guiflags = SGF_MULTISTRING
 def      = 2
-min      = 0
+min      = -2
 max      = 4
 str      = STR_CONFIG_SETTING_TOWN_GROWTH
 strhelp  = STR_CONFIG_SETTING_TOWN_GROWTH_HELPTEXT
-strval   = STR_CONFIG_SETTING_TOWN_GROWTH_NONE
+strval   = STR_CONFIG_SETTING_TOWN_GROWTH_EXTREME_SLOW
+orderproc = OrderTownGrowthRate
 
 [SDT_VAR]
 base     = GameSettings