Make modal iframe tab accessible

This is similar to this issue for Media in D7:

https://www.drupal.org/project/media/issues/2914241 →

The fix there added some Javascript to allow the tab to jump back up to the close button once focus got to the end of the iframe.

I'll see if I can implement something similar here!

This patch ports the JS from D7's media module, allowing the user to TAB out of the iframe, back to the close button on the modal.

I found an issue that only affected tabbing backwards. This latest patch fixes it!

New patch fixes another related issue: currently, if you TAB to the "Browse" button (the one that opens the modal) and press ENTER, the focus will momentarily move to the modal, and then be sucked back to the "Browse" button which is under the model.

With the patch, the focus will move into the modal (although, not in the most ideal place - it focuses the iframe itself, when it'd be better on it's first child or the modal's close button).

Alright, this is hopefully my last patch before getting some review :-)

In this version, it starts keyboard focus on the modal's close button when it opens, and allows focus to freely flow through the elements on the iframe and back up to the close button (both TAB'ing forward and SHIFT TAB'ing backwards).

I think this is everything necessary to say the modal iframe is tab accessible :-)

The last submitted patch, 11: entity_browser-keyboard-trap-3191302-11.patch, failed testing. View results →

Re-roll the patch #11 according to the latest code changes.