Draft: Custom colors in transparency checkerboard

I would find it useful to be able to set the colors of the transparency background checkerboard when silhouetting images.

I created a function gimp_checks_get_colors to use instead of gimp_checks_get_shades. I did not add this function to plug-ins/pygimp/gimpmodule.c and did not replace gimp_checks_get_shades in libgimpwidgets/gimppreviewarea.c. Please advise on whether I should.