Search_terms on the groups directory

This commit applies search term and order by selections to ajax group requests from the tree. So when you expand a parent group, the subgroups are filtered by search term and ordered as requested. (When the object is "tree" those jQuery selectors don't find the form inputs.) See also: https://buddypress.trac.wordpress.org/ticket/5679

I also made some logic changes to accommodate BP Group meta queries. 

Thanks for the great plugin! I hope I understood how the various filters were working… filtering the groups list hierarchically gets pretty complicated.