Add a filter to the usernames in wp_dropdown_users

[dominik.schwind]

At the moment the only way to change the output of the function is the wp_dropdown_users filter which gets the finished HTML code as a parameter.

I propose a new filter in this loop: https://core.trac.wordpress.org/browser/trunk/src/wp-includes/user.php#L1706 that allows a theme/plugin to hook into the creation of the $display value.

If there's no opposition to this idea, I'll happily work on this and create a patch.