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.