Allow exposing properties of objects other than XWiki.XWikiUser class in the "Profile" section of the user profile

Right now, the supported usecase for the "Users and Groups > User Profile" section in wiki Administration is for an admin to extend (or use) the XWiki.XWikiUsers class to store and expose extra (or existing) user properties/metadata/preferences. However, this usecase is not ideal when upgrading, since the XWiki.XWikiUsers class is a standard XE distribution document and it may cause conflicts as it needs merging.

Besides the upgrade issues, most applications already prefer to use their own classes for user preferences, so the Administration section can not be used to expose those preferences.

We need a way(syntax?) to also allow specifying the class of the property we are exposing in the "Profile" section of the user profile.

Example:
MyApp.MyClass^someProperty <- class prefix specified
someOtherProperty <- no prefix means XWiki.XWikiUsers as default