XmtDialogPosition() sets the XtNx and XtNy resources of
dialog so that when it is popped up it will appear centered over
the widget ref. If centering dialog over ref would cause
part of that widget to be off the edge of the screen, then
the position is adjusted so that all of dialog is visible.
Motif dialog boxes are automatically centered over their
ancestor shell widget, so you rarely will need to use this
function. It is useful when a single dialog box will be
popped up over more than one application window.