(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.

main_gui.c

@@ -1569,9 +1569,8 @@ static bool AnyTownExists(void)
 {
 	const Town *t;
 
-	FOR_ALL_TOWNS(t) {
-		if (t->xy != 0) return true;
-	}
+	FOR_ALL_TOWNS(t) return true;
+
 	return false;
 }