Implemented avg() window function.

It is based on the sum() function, thus much of the logic is shared.

Considering that there are 2 counters stored within the function, one
that handles the null value, while the other that handles the divisor
for the avg computation, it is possible to remove the counter from the
Item_sum_avg. I have not removed it in this patch as we may choose to
refactor the whole code into a separate class.
This remains to be dicussed.

sql/sql_window.cc

@@ -1585,6 +1585,7 @@ bool JOIN::process_window_functions(List<Item> *curr_fields_list)
           case Item_sum::COUNT_FUNC:
           case Item_sum::SUM_BIT_FUNC:
           case Item_sum::SUM_FUNC:
+          case Item_sum::AVG_FUNC:
           {
             /*
               Frame-aware window function computation. It does one pass, but