Introduce the wp-textbox xProfile field type's class.

BP_XProfile_Field_Type_WordPress_Textbox deals with the following WordPress usermeta keys, as a text input is used to fill their values:

• first_name,
• last_name,
• user_url,
• and potential WP contact methods.

It contains a specific metabox callback to let Admins select the WordPress usermeta key the field will specifically deal with.

Props DJPaul, Offereins, needle, netweb, vapvarun

See #7162