Popup-menu signals quit when Aborted

[JD Smith]

`x-popup-menu` is setup to handle mouse events specially when “aborting” the 
menu by clicking off of it:

> If the user gets rid of the menu without making a valid choice, for
> instance by clicking the mouse away from a valid choice or by typing
> keyboard input, then this normally results in a quit and
> ‘x-popup-menu’ does not return. But if POSITION is a mouse button
> event (indicating that the user invoked the menu with the mouse) then
> no quit occurs and ‘x-popup-menu’ returns nil.

This seems sensible.  But `popup-menu` "normalizes" all incoming positions 
using `popup-menu-normalize-position`, which strips any mouse event designation 
in the position before passing it on.  So there is no way via `popup-menu` to 
"indicat[e] that the user invoked the menu with the mouse”.  Hence it always 
quits and ring the bell when the user “gets rid of the menu".

Normalizing away mouse events in this manner seems like a bug in `popup-menu` 
to me, since `x-popup-menu` explicitly takes mouse events and behaves 
differently with them.

See https://github.com/tarsius/minions/issues/25 for original discussion.