Introduce a new configuration section in XWiki Administrator for Comparing Versions in History tab

One idea is to make the diff tabs UIXs of an UIXP. This would achieve several things at once:

• Add the ability to add more diff views (for ex from a contrib extension)
• If the UIXP provides a "isDefault" parameter or if we decide to display the 1st tab by default (based on an "order" parameter of the UIXP), then admins could configure which diff they decide to display by default/first.

Note that for the second point, this would still require some changes to some default page content (the UIX pages) but it would be more controlled and less subject to merge issues when XWiki is upgraded. An Admin UI could be also be added to control the default diff tab, by setting the UIX id of the default tab to display.