Add a group's schema field that can be displayed instead of groupname

UserManager: org.nuxeo.ecm.platform.usermanager.UserManager#searchGroups(String) signature changed, returns a DocumentModelList change some methods signature that takes HashSet to Set Add needed methods to manage GroupSearchFields JSF, two new functions to choose between group Id or Label: # {nxu:groupDisplayName(_ID_, _LABEL_)} , LABEL could be null # {nxu:groupFullName(_ID_)}