Set alpha for selected layer.

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

        # 2. Set alpha value for layer (if selected).
        if selected_iter is None:
            # No layer was selected, so disable scale widget.
            return
        else:
            surface_name, original_alpha = list_store[selected_iter]

            self.set_alpha(surface_name, alpha)
            self.set_scale_alpha_from_selection()