Add a framework for saving and restoring internal data objects, in
gimp-internal-data.c.  Internal data objects are saved in separate
files under a new "internal-data" subdirectory of the user's gimp
directory.  The internal data is saved, restored, and cleared
together with the tool options.

Use this to save and restore the custom gradient.  In the future,
we might add similar writable internal data objects that we'd want
to save.