bug#195: menu bar misbehavior during (ispell-buffer)

[Chong Yidong]

> When (ispell-buffer) is underway in one frame, and the displayed
> buffer in another frame has different/additional menu bar items,
> selecting that second frame doesn't update the menu bar.  However,
> clicking on the menu bar *does* trigger a change to the menu bar
> categories, which means you may click on the wrong item!  (i.e. the
> menu bar item under the mouse pointer may not be the same with or
> without mouse-1)

I checked in code to cause ispell to quit if the user enters a
frame-switch event or clicks in another frame.

This fix isn't perfect: it doesn't kick in if the user clicks directly
on a GTK menu (though it does cause quit when the user later leaves the
menu).  However, this may be a limitation of our GTK support, and OTOH I
don't see how to make it work.