WP_Comments_Table cleanup:

	* use column_*() methods
	* remove extra arguments from single_row()
	* introduce derived class WP_Post_Comments_Table for handling the post metabox comments
	* use $pending_count property instead of $_comment_pending_count global
See #14579



git-svn-id: https://core.svn.wordpress.org/trunk@15630 1a063a9b-81f0-0310-95a4-ce76da25c4cd