In GimpImageProxy, implement GimpColorManaged by forwarding the
functions to the underlying GimpImage, and forwarding the signals
in the other direction. This fixes color-managed view in the
Navigation dockable.
In GimpImageProxy, implement the GimpPickable interface, so that
the proxy can be used as both a viewable and a pickable for the
image projection, with direct control over the show-all mode. This
will allow us to use a GimpImageProxy as input for a GimpLineArt.