Non-public groups shouldn't appear in drop-downs during public session

[Rdornier]

When connecting to a public server via the Public user (i.e. checking the box Enable logging in as a public user) , all groups appear in the drop-down menu (both in the OMERO browser and in the advanced search) which, I think, should not be the case. Only public groups should be displayed in that case.

I also noticed that, in the drop-down list of users, the users who are not part of any public groups are displayed as <hidden><hidden>

For better lisibility, I think it could be nice to remove all <hidden><hidden>