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.
- gimp_container_tree_view_prepend_toggle_cell_renderer() to
gimp_container_tree_view_add_toggle_cell()
- gimp_container_tree_view_prepend_cell_renderer() to
gimp_container_tree_view_add_renderer_cell()
because "prepend" is an implementation detail, "renderer" is obsolete,
and in the second case it's not "cell renderer" but really a "renderer
cell".
Rename gimptoolview.[ch] to gimptooleditor.[ch]. The contents of the
file has already gone through this change, we do the file name change
separately for better diffs. Part of fix for bug #500930.
