Allow to enable / disable a UIX in the wiki administration

XWiki comes by default with a set of UIX that are used for various things : applications in the application panel, drawer items, …
When customizing the wiki, a developer might want to remove some of these default UIXs while not worrying about potential upgrade issues along with this removal.

One solution would be to build a configuration page, much like the one used for configuring loggers, that lists every UIX in the wiki and allows an administrator to enable or disable some of them. By default, the UIX are considered as enabled, but can be disabled if needed through this interface.