Wrapping div surrounding video shortcode cannot be reffered easily

[AlphaK]

In changeset [24789], I found it was very annoying having that div wrapping the video tag, without a class or anything to refer the div using CSS...
It makes for instance very difficult to center or to style the video without using javascript, as the wrapping div has to be targeted.

I thing adding a class to the div would greatly help.

[nofearinc]

adding a wp-video class to video shortcodes

[wonderboymusic]

In 26526:

Add a wp-video class to the div that wraps video shortcode output. It was previously hard to target.

Props nofearinc.
Fixes #25896.