This adds play/pause controls to video headers, along with voice
assistance, using `wp.a11y.speak`, to make custom video headers more
accessible. To make styling the play/pause button easier for themes,
CSS has been omitted from the default implementation.
This also includes a refactor of the `wp.customHeader` code to introduce
a `BaseHandler` class, which can be extended by plugins and themes to modify
or enhance the default video handlers.
Props davidakennedy, afercia, bradyvercher, joemcgill, adamsilverstein, rianrietveld.
Fixes#38678.
git-svn-id: https://develop.svn.wordpress.org/trunk@39272 602fd350-edb4-49c9-b593-d223f7449a82