Turn GimpPlugIn into the main factory for all proxies and keep the
main hash tables there. The hash tables keep the initial reference.

For each GimpProcedure::run(), have s "sub-factory" which hands out
proxies to the actual procedure code. Each run() has hash tables of
its own which hold additional references. When run() is done, get rid
of its hash tables and their references, *and* drop the main plug-in
reference counts from the global hashes if the proxies' refcount has
dropped to one.