Per @Jehan, this solution would prevent default icons from being loaded
in custom themes if they did not include their own version.
The only thing kept from !909 is the replacement of hardcoded strings
in a few files with constants defined in gimpicons.h.
Five icons in the Layer dockable were being replaced by GTK defaults at
runtime. A "gimp-" prefix was added so that GIMP's version would always
be used. A few dialogues were fixed to use constants rather than
hardcoding the filename, to make it easier to update the icon in the
future.
My last commit message was slightly wrong. I had one last special case to
handle: the ability to only create sub-GimpMenuShell (i.e. at a specific path
other than the root).
We are using this ability in particular for tool options submenus.
Alx Sa