Dash cleanup.
 * Use wp_add_dashboard_widget() rather than add_meta_box().
 * Use original functions like wp_dashboard_primary(), wp_dashboard_right_now(), wp_dashboard_quick_press() (where possible).
 * Only include plugins section of the news widget when appropriate, which necessitates the set_current_screen() via the ajax action.
 * Remove some debug cruft that was preventing caching and invalidation.
 * Simplify lots of things where possible.
see #25824.


git-svn-id: https://develop.svn.wordpress.org/trunk@26220 602fd350-edb4-49c9-b593-d223f7449a82