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.