It's just too weird to be public. Remove its properties from the wire
protocol and from pluginrc. Instead, have all GParamSpecs' flags on
the wire and in pluginrc, so we can use stuff like
GIMP_PARAM_NO_VALIDATE.
Port the remaining few places to GIMP_PROC_ARG_STRING().
I'm sure something is broken now wrt UTF-8 validation,
will add tighter checks in the next commit.
which means that it's now included normally via gimpbase.h
and not any longer via gimpbasetypes.h which we only did out
of lazyness. A *lot* of files in libgimp* and app/ now need to
all the stuff from app/core/gimpparamspecs.[ch] that is not about
image, drawable etc IDs, these will have to go to libgimp with
different implementations than in app/.