Try to sort all GIMP_ICON_* defines into FDO categories like in
https://specifications.freedesktop.org/icon-naming-spec/latest/ar01s04.html
Add defines for all icons we override, rename some icons to their FDO
standard names, and mark the ones we duplicate with a comment so we
don't forget to rename those to standard names in 3.0.
Nobody has them anymore, and they are deprecated in GTK+ 3.x. This
also fixes all conflicting mnemonics except those I missed, but we can
fix them now.
and call gimp_image_convert_indexed() in image-commands.c where it
belongs. Also remember the default values there, they will to to
GimpDialogConfig next.
They are unreliable because every type checking cast discards them,
they are useless anyway, visual clutter, added inconsistently, and
generally suck. Wanted to do this a long time ago, it was a bad idea
in the first place.