Changed the base class for Item_window_func from Item_result_field to

Item_func_or_sum.
Implemented method update_used_tables for class Item_findow_func.
Added the flag Item::with_window_func.
Made sure that window functions could be used only in SELECT list
and ORDER BY clause.
Added test cases that checked different illegal placements of
window functions.

sql/sql_priv.h

@@ -344,6 +344,7 @@ enum enum_parsing_place
   IN_WHERE,
   IN_ON,
   IN_GROUP_BY,
+  IN_ORDER_BY,
   PARSING_PLACE_SIZE /* always should be the last */
 };