I made an attempt at fixing the problem : in view_all_bug_page.php,
when sorting issues by category, it operates on category_id instead of
This is the first time that I go anywhere near the filters API, and make
changes to the code that affects the generated SQL, so I am not quite
sure if my approach is correct; basically, what I did is adding a join
on the categories table and use the name field for sorting.
It works, but I'm not sure it's kosher so I would appreciate if someone
with more experience in this area could have a look at my changes 
and make comments.
Thanks in advance