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.

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git-svn-id: http://core.svn.wordpress.org/trunk@26196 1a063a9b-81f0-0310-95a4-ce76da25c4cd