Adapt code from AquaEmacs to handle occasional blank menus.

Adapt code from AquaEmacs to handle occasional blank menus.

* nsmenu.m (trackingMenu): New variable.
(NSMenuDidBeginTrackingNotification): Declare if OSX < 10.5 and
NS_IMPL_COCOA.
(trackingNotification): New method (from AquaEmacs).
(menuNeedsUpdate): Expand comment and return if trackingMenu is 0,
from AquaEmacs.
(syms_of_nsmenu): Set trackingMenu to 1 if not NS_IMPL_COCOA.

* nsterm.m (ns_term_init): Subscribe for notifications
NSMenuDidBeginTrackingNotification and NSMenuDidEndTrackingNotification
to method trackingNotification in EmacsMenu.