[layers] Implemented layer insertion. No keys or menu yet, but it's already used when loading images, to make an image with right number of layers. Undoing/redoing such change works. Implemented single-layer deletion (untested)
git-svn-id: svn://pulkomandy.tk/GrafX2/branches/layers@1069 416bcca6-2ee7-4201-b75f-2eb2f807beb1
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