code: def build_gui(self, container):
          
          vbox = Widgets.VBox()
          vbox.set_border_width(0)
  
          w = Viewers.GingaViewerWidget(viewer=self)
          vbox.add_widget(w, stretch=1)
  
          # need to put this in an hbox with an expanding label or the
          # browser wants to resize the canvas, distorting it
          hbox = Widgets.HBox()
          hbox.add_widget(vbox, stretch=0)
          hbox.add_widget(Widgets.Label(''), stretch=1)
  
          container.set_widget(hbox)
docs: This is responsible for building the viewer's UI.  It should
          place the UI in `container`.  Override this to make a custom
          UI.