Deprecate stock items for good and change all icon defines to GIMP_ICON_*

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.

plug-ins/script-fu/script-fu-server.c

@@ -886,7 +886,7 @@ server_interface (void)
   gtk_box_pack_start (GTK_BOX (main_vbox), hbox, FALSE, FALSE, 0);
   gtk_widget_show (hbox);
 
-  image = gtk_image_new_from_icon_name (GIMP_STOCK_WARNING,
+  image = gtk_image_new_from_icon_name (GIMP_ICON_DIALOG_WARNING,
                                         GTK_ICON_SIZE_DIALOG);
   gtk_box_pack_start (GTK_BOX (hbox), image, TRUE, TRUE, 0);
   gtk_widget_show (image);