Add support for overriding the default WYSIWYG editor from request

Currently, you can configure the default WYSIWYG editor for a given user, subtree of pages, wiki and farm. The problem is that this makes it hard to test new, experimental, editors without affecting other users or your own productivity. It would be nice to be able to override the default editor for a single HTTP request, through the request parameters. This would allow an XWiki extension that provides a new editor to inject links in the Edit menu to quickly try the editor (i.e. to temporarily switch to the new editor for a single request).