Toolbar: Add search icon prefix to command palette item in admin bar.

Displays a search icon before the shortcut text in desktop view to clarify the item's purpose.

Follow-up to [61912], [61979].

Props sabernhardt, siliconforks, presskopp, westonruter, wildworks.
See #64672.

git-svn-id: https://develop.svn.wordpress.org/trunk@62092 602fd350-edb4-49c9-b593-d223f7449a82