GUI: improve the group implementation

fixes:
nodeChangedAndWait: Group (filesys_vip) node changed error:
java.lang.Throwable
        at lcmc.utilities.Tools.getStackTrace(Tools.java:452)
        at lcmc.gui.Browser.nodeChanged(Browser.java:190)
        at lcmc.gui.resources.Info.selectMyself(Info.java:452)
        at lcmc.gui.CRMGraph.oneVertexPressed(CRMGraph.java:896)
        at
lcmc.gui.ResourceGraph$MyGraphMouseListener$2.run(ResourceGraph.java:
1192)