In an authoring context, the image `alt` attribute purpose is different from the one for the front end. For example, screen reader users need to know what the selected image is, even when the original `alt` value is empty.
This change introduces a new pattern for the `alt` text in an authoring context:
- uses the `alt` text if not empty
- when there's no `alt` text, informs users the image has no alternative text and provides a reference to the image filename
Also, makes the gallery media widget preview an unordered list to make screen readers announce the number of images automatically.
Fixes#43137.
Built from https://develop.svn.wordpress.org/trunk@44767
git-svn-id: http://core.svn.wordpress.org/trunk@44599 1a063a9b-81f0-0310-95a4-ce76da25c4cd
adc7f9fe54