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

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The button does not disappear, but I do not think this should be expected behavior. (I am not talking about the buttons menu, but autocomplete that appears while typing, which is irrelevant to the menu and should be handled like the other leave events.

Even if you think moving the cursor over auto complete window should not make the button disappear, there is still work to do, because the button also remains when the cursor follows the path Button -> AutoComplete -> anywhere else, even when the button is _not_ clicked. This is a corner case but I think it is better to solve it completely.