This is heavily work in progress, posting now since Jacques is working on the
user interface side of this.

Generated images are now automatically converted to packed single images
the first time they are modified (by painting, baking, ..).

TODO: correctly mark image as dirty after undo/redo
TODO: don't pack immediately when painting on the image for the first time
TODO: set automatic filepath so unpack does something useful
TODO: auto unpack on first save?

Save all modified operator in C, replacing Python operator. Also has API in
ED_image.h to hook into .blend file save.

TODO: check valid filepath?