*  Example used in book

*xmenu1  Spring-loaded
*xmenu2  Pulldown, popped up with Command, Popup on Press.  Down on release outside.
*xmenu3  Pulldown, popped up with MenuButton instead of Command, using MenuButton popup code.  Down on release outside.
*xmenu4  Pulldown invoked by Command, with submenu, Using grab mode XtGrabExclusive
*xmenu5  Spring-loaded with submenu, more sophisticated CheckLeftAndPopdownSubmenu, using grab mode XtGrabNonexclusive.

xmenu6  Attempt at Pulldown identical to xmenu2 except using 
XtCallbackExclusive (callbacks only - no actions).  Doesn't work 
because XtCallbackExclusive calls XtPopup which can't pass the right 
parameters to _XtPopup.  Even if you write your own callback it still 
can't be made to work.  (Moral: this type of menu must use MenuPopup 
action.)  This is fixed by a new function in R4.

*xmenu7.c  using R4 SimpleMenu, MenuButton, and all three Sme gadgets.

xdialog  Test of XtCallbackExclusive callback for popping up a dialog
box.
