Custom query args for `search_box` form in `WP_List_Table`

[xipasduarte]

There is no way to preserve a custom query argument in the search forms built by the WP_List_Table class other than replacing the class that is used through the filter wp_list_table_class_name.

Replacing the class is effective and provides a lot of customization, when needed, but it's an overkill when we want to add a query argument that has no visible field, like the ones already supported. We'd need to create a custom class for every existing class extension of WP_List_Table, which would be very hard to maintain.

This proposal is for a new filter (search_box_custom_args) that will allow just that. Add a custom query argument as a hidden input to the form, when present.

Provide a filter to add custom query arguments as hidden fields on search forms in the admin interface listings.

Trac ticket: core.trac.wordpress.org/ticket/61899

[xipasduarte]

Add search_box_custom_args filter