Accessibility: Customizer, improve UI controls in customize.php

- 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.
Built from https://develop.svn.wordpress.org/trunk@37230


git-svn-id: http://core.svn.wordpress.org/trunk@37196 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/customize-controls.css

@@ -1550,10 +1550,11 @@ body.adding-widget .add-new-widget:before,
 		left: 48px;
 		line-height: 45px;
 		font-size: 14px;
-		padding: 0 12px 0 12px;
+		padding: 0 12px;
 		margin: 0;
 		height: 45px;
 		background: #eee;
+		border: 0;
 		border-right: 1px solid #ddd;
 		color: #444;
 		cursor: pointer;