GimpDisplay contains only the ID logic and the "gimp" and "config"
pointers, and lives in the core.

GimpDisplayImpl is a subclass and contains all the actual display
stuff. The subclass is only an implementation detail and doesn't
appear in any API.

Remove all hacks which pass displays as gpointer, GObject or
GimpObject through the core, or even lookup its type by name,
just use GimpDisplay.