Allow Custom Menu Edit Markup

[filosofo]

Menus need a way for plugin authors to customize their edit markup.

For example, with a plugin I want to create a menu option that changes descendant menu items automatically based on context. Currently, there is no way to alter the existing menu edit page to present my plugin's UI.

This patch allows plugin authors to filter the Walker class which handles the admin menu edit markup, so the markup can differ for each menu.

[filosofo]

I also corrected the documentation. (Proximate in the file, but unrelated).

[nacin]

Don't think we need the string message in WP_Error, but looks sane to me.

[filosofo]

Replying to nacin:

Don't think we need the string message in WP_Error, but looks sane to me.

I'm fine with its doing nothing instead, but I put that there since it already bubbles up error objects from retrieving the nav menu.

[ryan]

(In [14645]) Add wp_edit_nav_menu_walker filter for specifying a custom edit nav menu walker. Props filosofo. fixes #13342