Fix #187 by removing / readding menu_button on mouse_enter/leave events

See commit message
> There is also another case that should be handled separately, namely when the cursor leaves entering the auto complete window. In that case menu button remains regardless of the event, as the leave event is not triggered. Do you think enter event of the auto complete window would be a proper place to handle this?

What exactly needs to be handled in this case? Isn't the behavior already as expected (i.e. the button does *not* disappear)?