Introduce dashboard_glance_items filter which replaces 3.7 right_now_* hooks...

Introduce dashboard_glance_items filter which replaces 3.7 right_now_* hooks that were tied to table markup.

fixes #26495.

Built from https://develop.svn.wordpress.org/trunk@26832


git-svn-id: https://core.svn.wordpress.org/trunk@26719 1a063a9b-81f0-0310-95a4-ce76da25c4cd