Menus stay open after an item is clicked

[mxgrey]

Menus that drop down from the menu bar will currently remain expanded after one of its items is clicked. Sometimes this makes sense, like for toggle button items. But most of the time this is undesirable, like for Save as... or Export _ buttons.

We should either tweak the implementation so that buttons will immediately close the menu after being selected while toggles keep it open, or else allow the developer to indicate whether the item should automatically close the menu.