Don't show image lib when lua adds a button

This made dt to always show the selected image[s] lib on startup, no
matter what's the expanded state.

src/libs/image.c

@@ -370,7 +370,7 @@ static int lua_register_action(lua_State *L)
   data->key = strdup(key);
   data->self = self;
   g_signal_connect(G_OBJECT(button), "clicked", G_CALLBACK(lua_button_clicked), data);
-  gtk_widget_show_all(self->widget);
+  gtk_widget_show_all(button);
   return 0;
 }