Display name choice for nickname should update when nickname field is changed

[hansengel]

In wp-admin/profile.php, if the user wants to set their nickname and set the nickname as their display name, they must first update the options so that the new nickname will appear in the list of possible display names.

In this patch, jQuery is used to dynamically update the nickname in the display name list, so the user can see that the changes are immediate, and is not confused about what to do.

[hansengel]

Adds dynamic updating of nickname option in display name list with r6636

[hansengel]

Updated patch for r8133 - now removes the nickname option if it is empty

[westi]

(In [8135]) Update nickname in display name dropdown dynamically. Fixes #5691 props hansengel.