PATCH: improve sorting in the Data Editor

Hello,

Here is a small improvement about sorting in the Data editor.

Problem:
If a table has a PK or OIDs, it gets sorted by PK/OID by default.
When a user wants to sort by any other column, he has first to click
"Remove sort" in the context menu, and only then click "Sort
ascending"(descending) on column of his choice.

If he doesn't remove the default sort, he will get no effect and
probably be puzzled for the first time until guesses that he makes
  ORDER BY column_pk, column_of_choice
which is totally useless.
The need to "Remove sort" each time is annoying.

Solution:
When a user chooses to sort by some column, we must discard the
default sort order.

Please see the attached patch.
Thanks.

-- 
Best regards,
 J.F.