Add support for UIXP aliases

We need to quickly implement aliases for UIXP (I raised a few times on the list in the past and we see in https://github.com/xwiki/xwiki-platform/commit/1b6019d757b69f44107586cab2fad400efcbc9c2#commitcomment-15370307 that we need it).

It should be easy to implement:

• Add a new xproperty in the UIX XObject to list all aliases
• At startup, cache those aliases in a Map
• In UIExtensionManager.get(), search in the Map if no UIX is found with the passed id