Not possible to add custom display_name option

[xinne]

In The Netherlands we do have insertion names, to be placed in between the first and last name. It is possible to add a custom field during registration, but in order to give the user the option to include the insertion name in his display_name, a core modification is necessary.

Hereby the request to add a line to wp-admin/user-edit.php at line 422:

<?php
  $public_display = apply_filters( 'edit_user_public_display_dropdown', $public_display, $profileuser);

This way, it is possible to add custom public_display options.

[JPry]

First pass at adding filter and cleanup

[JPry]

I've added a patch that includes a new filter user_profile_public_display and some formatting cleanup of the surrounding code.

I would be up for adding unit tests if there is a particular example I should follow for filters.

[azaozz]

Related #6148. See the proposed solution there to have a single field for user's name. That will change how the display name works (this ticket) too.