Introduce `BP_XProfile_Query`.

Based on `WP_Meta_Query`, `BP_XProfile_Query` is a helper class for generating
queries against users' XProfile data. It allows developers to filter user
queries to those users who have profile data matching a wide range of
comparisons, from simple equality ("city=Chicago") to complex combinations of
various comparison operators.

An `'xprofile_query'` argument has been introduced to `BP_User_Query` to
support use of `BP_XProfile_Query` inside of `bp_has_members()` loops.

See #5839.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9178 cdf35c40-ae34-48e0-9cc9-0c9da1808c22