Fix unrelated help text tooltips if a popup is shown during the delay

Fix unrelated help text tooltips if a popup is shown during the delay

* doc/lispref/frames.texi (Pop-Up Menus): Document new hook.
* etc/NEWS: Announce `x-pre-popup-menu-hook'.
* lisp/tooltip.el (tooltip-mode): Make sure `tooltip-hide' is
run before any popup menu is displayed to prevent unrelated help
text from obscuring the popup menu if it pops up during the
tooltip delay.
* src/menu.c (x_popup_menu_1): Run said hook right before the
popup menu is displayed.
(syms_of_menu): New hook `x-pre-popup-menu-hook'.