Move nav menus to a tabbed interface, so that users with a lot of locations can do bulk assignment.

props DrewAPicture, lessbloat. fixes #23770

git-svn-id: http://core.svn.wordpress.org/trunk@23810 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Minor revisions PHP reorg, code cleanup, restores _post_restored_from functionality.

props adamsilverstein. see #23497

git-svn-id: http://core.svn.wordpress.org/trunk@23811 1a063a9b-81f0-0310-95a4-ce76da25c4cd