Add minimum town distance patch, with minor modifications

Move implicitly added value of 20 to setting default.
Adjust setting texts.

From: https://www.tt-forums.net/viewtopic.php?p=1095146#p1095146

src/town_cmd.cpp

@@ -1800,7 +1800,7 @@ static CommandCost TownCanBePlacedHere(TileIndex tile)
 	}
 
 	/* Check distance to all other towns. */
-	if (IsCloseToTown(tile, 20)) {
+	if (IsCloseToTown(tile, _settings_game.economy.town_min_distance)) {
 		return_cmd_error(STR_ERROR_TOO_CLOSE_TO_ANOTHER_TOWN);
 	}