- makes the mobile preview/customize toggle a button
- changes the "Close" link hidden text from 'Cancel' to 'Close the Customizer and go back to the previous page'
- adds a missing `type="button"` attribute
- removes unnecessary `keydown` events and `preventDefault()`: buttons don't need a keydown event and when they have a `type="button"` attribute there's no default action to prevent
Props Cheffheid, afercia.
Fixes#31487.
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git-svn-id: http://core.svn.wordpress.org/trunk@37196 1a063a9b-81f0-0310-95a4-ce76da25c4cd