Add filters to group list

This is mostly for you @apeisa, as I've already explained this to @niklaka :)

I'd like to add simple filtering feature to groups view. Simple, because by default it's nothing more than text search, but again extendable by hooking into filter form and/or selector creation. This whole thing is (intentionally..) very similar to that found from Process User Extended.

If this sounds (and looks) good, feel free to accept pull request.