Start copying all the actual wire communication to GimpPlugIn, and
move the legacy versions to gimplegacy.c.
This implies having the entire protocol code twice, but without any
if(PLUG_IN) { plug_in_stuff(); } else { legacy_stuff(); }
At the moment it is a wild mixture of old and new, but when finished
the wire code in gimplegacy.c will be entirely separate from the wire
code in GimpPlugIn, which will make it easy to g_assert() that only
one API is used by a plug-in.