When the alt text for a media item is present in the media library, that
value should be copied into the Image block when inserted. This behavior
avoids the need to re-enter the alt text for each image inserted into
the post content.

Previous code required an alt text value, which broken existing code.
This overrides the method to support both contexts. It also updates
existing code to use the same method of generating media throughout the
source.

Consistently setting the alt text for all platforms will likely help
avoid confusion.

The demo editor now returns an alt text for the test image media.