MCE View sandboxes:

* Use a `MutationObserver` to listen to the `body` class of the parent editor frame.
* In `wpview_media_sandbox_styles()`, only return the MEjs stylesheets.
* In `wp_ajax_parse_media_shortcode()` and `wp_ajax_parse_embed()`, return an object instead of an HTML blob to allow passing `body` and `head` separately	

Props avryl, azaozz.
Fixes #29048.

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