Live update the menu item title when the user is editing the "Navigation Label" field.

Props celloexpressions.
See #23076.

Built from https://develop.svn.wordpress.org/trunk@28707


git-svn-id: http://core.svn.wordpress.org/trunk@28523 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add a function: `wp_spaces_regexp()`. It is not used yet.

This function is required by a bunch of patches by miqrogroove. Needs filter docs.

Props miqrogroove.
See #27588.

Built from https://develop.svn.wordpress.org/trunk@28708


git-svn-id: http://core.svn.wordpress.org/trunk@28524 1a063a9b-81f0-0310-95a4-ce76da25c4cd