Hooks for xprofile Add Field screen.

Until this screen uses the add_meta_box() API, these hooks will allow plugin
devs to add custom items to the xprofile field creation process.

Props svenl77.
Fixes #5917.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9075 cdf35c40-ae34-48e0-9cc9-0c9da1808c22