Override ->single_row_columns() in WP_Posts_List_Table.

Break apart the giant `switch` statement in `->single_row()` into `column_{$column_name}` methods.
To maintain the `->single_row_columns( $item )` interface, add a property, `$current_level`, to allow access to `$level`.

This list table class is now easier to subclass.

Props joehoyle, wonderboymusic.
See #29881.

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git-svn-id: http://core.svn.wordpress.org/trunk@32711 1a063a9b-81f0-0310-95a4-ce76da25c4cd