Implementation of OpenRaster layered image file. It seems to be fully
compatible with most images out there (except for ones with layer
offsets, but Spade doesn't have virtual area to hold layers off the
clipping screen)

I strongly recommend abolishing the goofy tree-style layers menu in
favor of a more traditional list one. Why does it even exist?
Merge pull request #3 from SuperDisk/master

Implementation of OpenRaster layered image file.