Change plug-in-exchange arguments to types Color and Float instead of Uchar
This plugin is a "compatibility" procedure in the PDB that wraps the gegl plugin gegl:exchange.
Some args are declared type Uchar which is not supported in GimpProcedureConfig. So calling the plugin throws an error. Type Uchar was used to pass 8-bit RGB values of a color, and so forth.
The gegl plugin has a more natural signature using Color and Float between 0 and 1.
Since it doesn't work anyway, and since we don't want to support Uchar in the signatures of PDB procedures, we should obsolete the old signature, but keep the procedure in the PDB In case some third party scripts want it, but they will need to be ported, i.e. this is API breaking. I would guess that there are next to none third party scripts that will be affected.
We don't want to support Uchar in signatures of PDB procedures for the same reason we eliminated INT16: we don't need to quibble about a few bytes when passing args. Also, we used to support INT8_ARRAY types, but that was replaced by GBytes, for the use case where size does matter.
This change does not affect the GUI of GIMP Color>Map>Color Exchange, since the gegl plugin is called directly, not the PDB wrapper.
Related to #10833 (closed) and !1335 (merged)