radix-modal.js breaks CTools accessibility changes from #2280853

radix-modal.js monkey patches CTools to use the Bootstrap modal, which also cause the CTools accessibility changes from #2280853: Keyboard trap when using ctools modal to no longer work.

You can see this by opening the "Add content" dialog in the IPE: you'll see that the close button is briefly focused (which is correct) and the focus is moved to the first category button (which is incorrect).

Essentially, CTools is looking for a specific class on a specific element to determine if the CTools model is being loaded for the first time (which means we should focus on the close button) or if the modal is already open and it's content is being replaced (which means we should focus on the first focusable child).

But CTools isn't finding the class, because (a) the markup for the modal has changed AND (b) the replacement code for Drupal.CTools.Modal.show() isn't adding the class in the first place. This is complicated by the fact that both CTools and Bootstrap use the class "modal-content" but for slightly different things.