We don't try anymore to convert early from a pickable color to another
format/space. Now we are able to get a GeglColor and move it around,
doing only last-second (when needed) conversions.
Replaces GimpPickableInterface's pixel_to_srgb () functions with
pixel_to_rgb(). Now GimpRGB's values should be in the correct
image color space from the beginning of the process.
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.
