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...)