Allow easy registration of taxonomy columns on post (and custom post type) list table screens.

To register a column for a list table, use the new manage_taxonomies_for_{$post_type}_columns
filter. Introduces show_admin_column => true for register_taxonomy(), which automatically
displays that column on all associated post types.

props jtsternberg, SergeyBiryukov for initial patches.
fixes #21240.



git-svn-id: https://develop.svn.wordpress.org/trunk@21788 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/taxonomy.php

@@ -47,6 +47,7 @@ function create_initial_taxonomies() {
 		'rewrite' => $rewrite['category'],
 		'public' => true,
 		'show_ui' => true,
+		'show_admin_column' => true,
 		'_builtin' => true,
 	) );
 
@@ -56,6 +57,7 @@ function create_initial_taxonomies() {
 		'rewrite' => $rewrite['post_tag'],
 		'public' => true,
 		'show_ui' => true,
+		'show_admin_column' => true,
 		'_builtin' => true,
 	) );