Add progress bar and cancel button (6 gb RAM lol)

Program consumed 6 GB RAM and began paging memory to disk like crazy which made the whole system sluggish.
If you can't stitch the images correctly, downscale the resulting image to a reasonable size so it won't need that much memory or ask the user to give some hints.
Also it would be nice to have an option to only extract the subimage that is not black or "magic fill" the black regions.
Paper: http://gfx.cs.princeton.edu/gfx/pubs/Barnes_2009_PAR/index.php