Make XWiki#getDocument(EntityReference) support any kind of reference

Right now it just assume the reference is a document reference that may or may not be complete. But it would be nice to be able to pass any reference an let it find the corresponding document reference before loading the document (extract it when you pass an object reference, complete a space reference with the default space home page, etc).

Also we should make XWiki#getDocument(EntityReference) public, api.XWiki only expose XWiki#getDocument(DocumentReference).