There was a /* FIXME MOVE TO PRIVATE */ and anyway it makes sense to not
leave such data in the public API.

I note that the whole widget declaration is between #ifndef
GIMP_DISABLE_DEPRECATED macros so maybe we should just delete it
altogether for GIMP 3, but it might still have a usage. Maybe it could
also be interesting to experiment with the file portal on such widget
for plug-in usage? Let's see.