Set scale marker to alpha for selected layer.

def set_scale_alpha_from_selection(self):
        '''
        Set scale marker to alpha for selected layer.
        '''
        #  1. Look up selected layer.
        selection = self.treeview_layers.get_selection()
        list_store, selected_iter = selection.get_selected()

        # 2. Set adjustment to current alpha value for selected layer (if any).
        if selected_iter is None:
            # No layer was selected, so disable scale widget.
            self.adjustment_alpha.set_value(100)
            self.scale_alpha.set_sensitive(False)
            return
        else:
            surface_name, alpha = list_store[selected_iter]
            self.adjustment_alpha.set_value(alpha * 100)
            self.scale_alpha.set_sensitive(True)