Menu Items should inherit the properties of their original object

[filosofo]

If you create a menu item for, e.g., a page, it should use the page's title unless the title is explicitly set in the menu edit admin.

URLs stay synced with the original page's.

Other properties such as descriptions are independent of the original object.

Blanking out the title resets it to sync with the original object.

All this from an IRC discussion involving DeannaS, kgraeme, jane, and ryan.

Discussed possible additional features include:

• ajaxy edit of otherwise fixed fields (like slug on post page)
• UX change indicating which fields are editable and which are synced.

(In [14283]) More nav menu fixes. props filosofo. see #13148. fixes #13155, fixes #13157, fixes #13138, see #13134.