bug#4930: Fix for timestamp related popup-menu problem with some X servers

[Chong Yidong]

> Anyway, to cut the story short, below is a patch to fix this problem.
> It uses last_event_timestamp from keyboard.c instead of the timestamp
> From the button event.  AFAICT, last_event_timestamp is the timestamp
> From the button event, most of the time anyway, because usually the
> button event that will lead to the popup-menu being displayed is the
> last one that was processed.

Thanks.  I've checked your patch into the emacs-23 branch.