Move gegl_sampler_compute_scale to gegl-buffer.h
Make gegl_object_set/get_has_forked functions

And move them to a semi-public location instead of gegl-node.h
Add gegl-operation-context-private.h

Move the actually private parts of OperationContext to their
own header, in preparation for making gegl-operation-context.h
a public header.
Add gegl-extension-handler-private.h
Use public headers in gegl-plugin.h

Replace duplicate definitions with public headers.