Add a specialized propgui constructor for gegl:color-to-alpha-plus.
This op is currently in the workshop, but is set to be merged with
the existing gegl:color-to-alpha, so we omit the '-plus' from file-
and function-names.

The new op adds a pair of properties to control the radii, relative
to the selected color, below which colors become fully transparent,
and above which colors remain fully opaque.  Allow these properties
to be set by picking a color from the image, and calculating the
radius accordingly.