(popup_get_selection): Now static. New arg DO_TIMERS.

(popup_get_selection): Now static.  New arg DO_TIMERS.
If it is non-nil, run timers.  Use an unwind-protect to requeue
the events that were read ahead.
(popup_get_selection_unwind): New subroutine.
(popup_get_selection_queue): File-scope variable now holds that queue.
(xmenu_show): Pass 0 for DO_TIMERS to popup_get_selection.
(xdialog_show): Pass 1 for DO_TIMERS to popup_get_selection.
Use an unwind-protect to pop down the dialog box.
(xdialog_show_unwind): New subroutine implements that.