Feature: allow setting a custom terrain type to define highest peak

At least, TGP will try to reach it. It heavily depends on the map
if it is reachable at all. But for sure it will do its atmost to
get there!

src/settings_type.h

@@ -309,6 +309,7 @@ struct GameCreationSettings {
 	byte   water_borders;                    ///< bitset of the borders that are water
 	uint16 custom_town_number;               ///< manually entered number of towns
 	byte   variety;                          ///< variety level applied to TGP
+	byte   custom_terrain_type;              ///< manually entered height for TGP to aim for
 	byte   custom_sea_level;                 ///< manually entered percentage of water in the map
 	byte   min_river_length;                 ///< the minimum river length
 	byte   river_route_random;               ///< the amount of randomicity for the route finding