First pass at Profile Settings for improved profile field visibility UX:

* Adds `/members/single/settings/profile.php` template to `bp-legacy` and `bp-default`.
* Introduces `bp-xprofile.settings.php` for Settings specific functionality going forward.
* Introduces actions & screens for handling output and saving of profile visibility data.
* Introduces `_is_` function for identifying the new component/action URL.
* See #5352. (still more @todo...)

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7960 cdf35c40-ae34-48e0-9cc9-0c9da1808c22