code: def build_gui(self, container):
          
          vbox = Widgets.VBox()
          vbox.set_border_width(2)
          vbox.set_spacing(1)
  
          w = Viewers.GingaViewerWidget(viewer=self)
          vbox.add_widget(w, stretch=1)
  
          # set up to capture cursor movement for reading out coordinates
  
          # coordinates reported in base 1 or 0?
          self.pixel_base = 1.0
  
          self.readout = Widgets.Label("")
          vbox.add_widget(self.readout, stretch=0)
  
          #self.set_callback('none-move', self.motion_cb)
          self.set_callback('cursor-changed', self.motion_cb)
  
          # 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`.