menubar: re-open submenu when returning to it after hover on menu-less item

Per @jzaefferer:

> Something I didn't write down on the wiki page,
> but seems like it should get adressed: When
> clicking to open a submenu, hovering the
> non-menu-item closes the menu, and another hover
> on the closed menu should probably reopen it,
> instead of requiring another click. When it gets
> closed by a click, it should require the click to
> reopen. If that makes sense to you to, we can put
> it on the wiki.