* app/plug-in/plug-in-rc.c: implement a new pluginrc flag,
"has-init", so that only plug-ins with init functions are
initialized.
* app/plug-in/plug-in.ch
* libgimp/gimp.c
* libgimpbase/gimpprotocol.ch: send a new gimpwire message,
GP_HAS_INIT during the query stage if the plug-in needs to be
initialized. Only invoke the plug-in in init mode if the plug-in
has an init function.