Column headers revamp:

	* undeprecate get_column_headers()
	* undeprecate get_hidden_columns()
	* in WP_List_Table, rename get_column_headers() to get_column_info()
	* remove $taxonomy argument from single_row()
	* merge manage_columns_prefs() into screen_meta()
Fixes #14952. Fixes #14776.


git-svn-id: https://core.svn.wordpress.org/trunk@15653 1a063a9b-81f0-0310-95a4-ce76da25c4cd