A private FcObjectGetSet() is implemented that provides an
FcObjectSet of all registered elements.  FcFontSetList() is
then modified to use the object set from FcObjectGetSet() if
provided object-set is NULL.

Alternatively FcObjectGetSet() can be made public.  In that
case fc-list can use that as a base if --verbose is included,
and also add any elements provided by the user (though that has
no effect, as all elements from the cache are already registered).
Currently fc-list ignores user-provided elements if --verbose
is specified.