Tweak the Nav Menu UX, especially around menu creation.

props lessbloat, DrewAPicture, jkudish. fixes #23641

git-svn-id: http://core.svn.wordpress.org/trunk@23622 1a063a9b-81f0-0310-95a4-ce76da25c4cd
RTL for Nav Menu tweaks. see #23641. fixes #23645. props DrewAPicture.

git-svn-id: http://core.svn.wordpress.org/trunk@23623 1a063a9b-81f0-0310-95a4-ce76da25c4cd