Introduces `WP_Customize_Manager::get_previewable_devices()` with a `customize_previewable_devices` filter to change the default device and which devices are available for previewing. This is a feature that was first pioneered on WordPress.com.
Props celloexpressions, folletto, valendesigns, westonruter, welcher, adamsilverstein, michaelarestad, Fab1en.
Fixes#31195.
git-svn-id: https://develop.svn.wordpress.org/trunk@36532 602fd350-edb4-49c9-b593-d223f7449a82
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