which are GimpProcedure subclasses with API to register as load/save
handlers and their own kind of run functions that get their standard
arguments passed directly instead of packed into a GimpValueArray.

They also register their standard arguments themselves, which removes
quite some boilerplate from load/save plug-ins.

Remove gimpprocedure-private.[ch] because install() and uninstall()
are now virtual functions of GimpProcedure.