(svn r6047) -Codechange: FOR_ALL now _only_ loops valid items, and skips invalid ones

-Codechange: use IsValidXXX where ever possible
  Note: both changes to prepare for new pool system, which needs those changes.
  For every pool there are 2 ugly lines, which will be removed when done
  implementing new pool system.
  Based on FS#13 by blathijs, partly implemented.

town_gui.c

@@ -414,9 +414,7 @@ static void MakeSortedTownList(void)
 	if (_town_sort == NULL)
 		error("Could not allocate memory for the town-sorting-list");
 
-	FOR_ALL_TOWNS(t) {
-		if (t->xy != 0) _town_sort[n++] = t;
-	}
+	FOR_ALL_TOWNS(t) _town_sort[n++] = t;
 
 	_num_town_sort = n;