libabc: Make things hold a reference to their context

The sample libabc includes functions to get a "thing", as a sample
sub-object of the overall library context.  Each "thing" has a reference
to the parent library context, and a function to return that reference.
Given that, abc_thing_new_from_string should call abc_ref, and
abc_thing_unref should call abc_unref when freeing a thing.