Users will be able to override icons by saving files to
DATAFILES/icons/ eg ~/.config/blender/2.80/datafiles/icons/
which probably must be created first.
Files use fixed names blender_icons.16.png and blender_icons.32.png
and conform to current dimensions as compiled in the binary. No
need of having both files, fallbacks to compiled in version(s).
Wrong size or unknow file type will be reported, so put tiny PNGs
and you will see the expected sizes easily.