- app: gimp_context_get_(foreground|background)() are now returning a GeglColor.
- libgimp: PDB functions named similarly in libgimp are returning a newly
  allocated GeglColor too.
- A few other PDB functions (the ones using these functions) were updated and
  their signature changed to use GeglColor too, when relevant. Plug-ins which
  use any of the changed libgimp functions were fixed.
- GimpContext: signals "(foreground|background)-changed" are now passing a
  GeglColor.
- libgimpconfig: new macro GIMP_CONFIG_PROP_COLOR using gegl_param_spec_color().
- GimpContext: properties "foreground" and "background" are now GeglParamColor
  properties.
- app: All code interacting with GimpContext objects were updated to receive a
  GeglColor (that they may still convert, or no, to GimpRGB for now).
- app: gimp_prop_gegl_color_button_new() was added as an alternative to
  gimp_prop_color_button_new() when the property is a GeglParamColor. Eve...