pmpro-buddypress field sync support in GUI

[DarkLotus]

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Changes proposed in this Pull Request:

Brings across the abilty to enable Syncing PMPRO Fields to BP XProfile fields via the GUI instead of just via register helper code defined fields.
Adds a new input thats only visible in the user field settings if pmpro-buddypress is enabled, that lets you set the $field->buddypress value via GUI.

Other information:

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Changelog entry

Add support for GUI user fields to support BP sync if pmpro-buddypress is installed.