Accessibility: custombox does not trap focus

[xaddict]

Custombox misses some Accessibility enhancements.

For example, dialogs/modals/alert pop-ups should trap focus so users can close the modal without having to tab through all the links on the page that opened the modal:

https://www.w3.org/TR/wai-aria-practices/#dialog_modal
https://www.w3.org/TR/wai-aria-practices/#alertdialog
https://edenspiekermann.github.io/a11y-dialog/
https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/ARIA_Techniques/Using_the_alertdialog_role
https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/ARIA_Techniques/Using_the_dialog_role