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.
actions/Makefile.am
actions/actions.c
dialogs/dialogs-constructors.c
dialogs/dialogs-constructors.h: further change on menu work.
actions/dynamics-editor-actions.c
actions/dynamics-editor-actions.h (new files): add actions for dynamics.