Customizer: Add screen reader text to announce that panels and sections are expandable.

props joedolson, ocean90.
fixes #27591.
Built from https://develop.svn.wordpress.org/trunk@29240


git-svn-id: http://core.svn.wordpress.org/trunk@29024 1a063a9b-81f0-0310-95a4-ce76da25c4cd