icons: Changes in color-scalable & scalable

new in color-scalable:
gimp-convert-grayscale
gimp-convert-rgb
gimp-convert-indexed

changed in scalable (to be in sync with color):
gimp-convert-grayscale
gimp-convert-rgb
gimp-convert-indexed

Signed-off-by: klausstaedtler <staedtler-przyborski@web.de>