Re-enable menu auto-closing now that some obstacles to that have been fixed

@dinhviethoa

I've fixed some small issues with the built-in autoclosing capability of `<spark-overlay>` in #774 (which are anyway unimportant now that the +/- buttons are gone from the menu). With that, menu autoclosing is trivial to achieve with a few lines of code in spark_menu_button.*. I think this fix is simpler than #755, better utilizes the already available functionality, and avoids dependency of spark-menu-button on an external element with fixed name and z-index. It also doesn't block the UI, so buttons can be clicked or files in the treeview selected, for example.