Follow on from #21240. Add sortable to taxonomy column

[tifosi]

Following on from the #21240 ticket which introduced the show_admin_column functionality one limitation is the 'as easy' way to make that column sortable.

It's possible to do it through the 'manage_edit-{$post_type}_sortable_columns' filter but the column_key is different for 3.5+. A standardisation as for adding the column may be advantageous: a register_taxonomy argument and / or new filter.

argument: show_column_sortable, boolean default false, requires show_admin_column to be set and true.

In the same way that the get_columns function uses the new "manage_taxonomies_for_{$post_type}_columns", filter to add a taxonomy column there could be a "manage_taxonomies_for_{$post_type}_sortable_columns" filter in the get_sortable_columns function of the post-list class:

i.e.

function get_sortable_columns() {
	$default = array(
		'title'    => 'title',
		'parent'   => 'parent',
		'comments' => 'comment_count',
		'date'     => array( 'date', true )
	);

	return apply_filters( "manage_taxonomies_for_{$post_type}_sortable_columns", $default );
}

The developer could then set the filter as normal

add_filter( "manage_taxonomies_for_{$post_type}_columns", 'add_sortable_column' );

function add_sortable_column ( $columns ) {
	$columns[ taxonomy-tax_name ] = 'Taxonomy Label';
	return $columns;
}