Adding a command to the Map Window Context Menu

A menu Item can either be directly added to the SIS Map Window context menu or to a pre-existing Menu Item object as a nested sub-menu.

        # Create the parent menu item
        self.parent_menu = gislink.MenuItem("My Context Menu Parent")
        # Add menu item to the context menu
        # Create the nested menu item
        self.nested_menu = gislink.MenuItem("My Nested Menu")
        # Add nested menu item to parent menu item, rather than root context menu