improved installer to avoid install of Gem libraries when doing incremental install (should significantly shorten incremental installs)

improved ggee/image to have a default empty image and ensured that its layer ordering is preserved after loading a new image. Finally, made a check for valid images--if invalid format, the image reverts to default empty image included in the ggee.

updated Makefiles for merged libraries, improved gem2pdp config file hopefully fixing Fero Kiraly's build error. Updated pd-l2ork-TODO.txt file reflecting newly synced externals.