phpdoc tweaks
Deprecate the xprofile_field_additional_options filter
Introduce new abstract function to output HTML for a field type on the wp-admin Profile Fields "Edit" screen.

This will eventually replace everything `render_admin_form_children()`.
Make `admin_edit_field_html` not abstract + update phpdoc

The method doesn’t need to be implemented for all field types. It's
used in core by the selectbox, multi selectbox, checkbox, and radio
button fields, to allow the admin to enter the child option values
(e.g. the items in the select box).
Don't call render_admin_form_children any more
Don't recreate type_obj in `BP_XProfile_Field` if it already exists when the field is `save()`d.
Always re-create $type_obj, in case the ->type is changed via a filter
If `BP_XProfile_Field_Type` is created by instantiating a `BP_XProfile_Field`, make sure each object has a reference to the other.
Rename some actions
Split the sorting part of `render_admin_form_children()` into a helper method
Remove redundant brackets
Spluit options handling in `render_admin_form_children()` into helper method
Add more sanitisation