Hookable filters (and other goodies)

Couple of changes and/or additions:

* First of all, filters (creating filter form and building selector) made hookable so that external modules can modify them if needed. I've actually got an implementation of group filter (with a new hook from UserGroupsHooks) working locally based on these changes :)
* User search / filtering didn't seem to work for me at all. Now it's functional.
* Some queries to $this->pages changed to $this->users.