Window functions: Better class names

As discussed on the call:
- s/Window_funcs_computation_step/Window_funcs_computation/g
- s/Window_func_sort/Window_funcs_sort/g

sql/sql_select.h

@@ -428,7 +428,7 @@ typedef struct st_join_table {
     Non-NULL value means this join_tab must do window function computation
     before reading.
   */
-  Window_funcs_computation_step* window_funcs_step;
+  Window_funcs_computation* window_funcs_step;
 
   /**
     List of topmost expressions in the select list. The *next* JOIN TAB