(svn r15695) -Feature [FS#2672]: Allow the number of towns that will be generated in the generate world window to be customized.

Some warnings: 
-the maximum number of towns to be accepted is set to 5000
-the minimum number of towns to be accepted is set to 1
-the number that is specified is NOT guaranteed to be the exact number of towns generated.  The normal mechanism of town creation has not been modified.  So town placement can still fail.
-setting a custom number of town will change your difficulty settings to custom as well

src/settings_type.h

@@ -156,6 +156,7 @@ struct GameCreationSettings {
 	byte   landscape;                        ///< the landscape we're currently in
 	byte   snow_line;                        ///< the snowline level in this game
 	byte   water_borders;                    ///< bitset of the borders that are water
+	uint16 custom_town_number;               ///< manually entered number of towns
 };
 
 /** Settings related to construction in-game */