Unpickling requires the class to be known, so we can't simply use to_dict()
to pickle and pass the dict to __init__() to unpickle. This works for the
pickled object itself (as pickle restores its type), but fails to restore
the class of subobjects, such as some_node.picture. This is why the code
now pickles each subobject too.
Python randomizes dict item order with a random seed, which changes every
time Python/Blender is run. As a result, Pillar GET parameters on the URL
would change, making them uncachable. Sorting the parameters solves this.
