Customize: Collapse available nav menu items panel when clicking outside over...

Customize: Collapse available nav menu items panel when clicking outside over preview or over existing items.

Props rockwell15.
Fixes #38953.

Built from https://develop.svn.wordpress.org/trunk@39548


git-svn-id: https://core.svn.wordpress.org/trunk@39488 1a063a9b-81f0-0310-95a4-ce76da25c4cd