Provides a popup menu so that when the right mouse button is pressed in the main
window a menu appears. The menu does not appear when the mouse points to a
control within the main window.
The clearwin_info@ parameters, POPUP_X and POPUP_Y reflect the position of
the mouse at the point at which %pm was invoked. These
values are screen coordinates before any subsequent movement to
select a particular menu item. They can be used in a user function
that is called when a popup menu item is selected.