Adding guest visibility to avoid to use groups like local.0 and a group ...

...local.X depending on insertion order in database

It's an improvment that we use to load the portlet in guestMode. To use it simply define a context for the guest user and add only in the visibility type (allowed/obliged...) the node <guest/>

We save several seconds in welcome page loading of our portal (usefull when you have large groups).

More it avoids to define the visibility on a local group which the id is database init order dependent.