Add an `end of DOM` UIXP

Modals (dialogs that do get all the focus until resolved) do not fit semantically anywhere on the page. If they are properly implemented (understand, implemented with the dialog HTML element and `showModal` javascript function), they won't have any semantics anyways.
In order for:

• the page to provide a nicer experience on legacy browsers / text only browsers
• the modal elements to not interract in a meaningless way with the semantics of other elements (see XWIKI-22996)
• Make the DOM a bit cleaner / easier to maintain and debug

We should probably add an UIXP to move all of these modals together.
This improvement was not voted, but talked about on synchroneous chat with mflorea and surli when trying to find a correct solution to XWIKI-22996 .