Customizer panels: use a single back button for all panels.

This also ensures the back button is visible on iOS devices.

props celloexpressions.
fixes #29135.

Built from https://develop.svn.wordpress.org/trunk@29610


git-svn-id: http://core.svn.wordpress.org/trunk@29384 1a063a9b-81f0-0310-95a4-ce76da25c4cd