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

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Partly, it works when doing drag-clicks and click-drags (both result in a disappearing button). I can not test touch event because  I do not have a touch screen and do not know how to simulate them (I don't think it is complicated) , and more importantly, I am not quite sure about the expected behavior on multitouch events.

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?