Introduce filter 'bp_xprofile_get_hidden_field_types_for_user'

This will make it easier for plugin authors to create custom visibility levels.

See #5317

Props megainfo

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7730 cdf35c40-ae34-48e0-9cc9-0c9da1808c22