Customize: Prevent re-highlighting "Add Items" button after available nav menu...

Customize: Prevent re-highlighting "Add Items" button after available nav menu items pane has already been opened.

Amends [41930].
Props bpayton.
See #42114.

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git-svn-id: https://core.svn.wordpress.org/trunk@41784 1a063a9b-81f0-0310-95a4-ce76da25c4cd