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: http://core.svn.wordpress.org/trunk@21788 1a063a9b-81f0-0310-95a4-ce76da25c4cd