Use of GadTools gadgets requires some per-window context information.
CreateContext() establishes a place for that information to go. This
function must be called before any GadTools gadgets are created.

    struct Gadget *CreateContext( struct Gadget **glistptr );

The glistptr argument is a double-pointer to a Gadget structure.  More
specifically, this is a pointer to a NULL-initialized pointer to a Gadget
structure.

The return value of CreateContext() is a pointer to this gadget, which
should be fed to the program's first call to CreateGadget().  This pointer
to the Gadget structure returned by CreateContext(), may then serve as a
handle to the list of gadgets as they are created.  The code fragment
listed in the next section shows how to use CreateContext() together with
CreateGadget() to make a linked list of GadTools gadgets.