Gets called on all unset key mappings. Arguments are: key combo and
current file. Thanks to Francesco Orsenigo (xarvh) for the idea.

Previously, the pointer would get warped when it had left the window.
This doesn't work when sxiv is run in fullscreen mode (or similar -- see
dwm's monocle mode): The pointer can never leave the window in such
situations.

To fix this, warp the pointer when it's on the first/last pixel. To
avoid an endless warping loop, the new position must be at least one
pixel away from the next warp position.

Also removed now obsolete -p command line option; fixes issue #98

Fallback to print all files, if no images are marked, was kind of
irritating.

- Command it_toggle_image_mark (bound to 'm') toggles mark of current
  image
- Command it_navigate_marked (bound to 'N'/'P') can be used to go to
  the next/previous marked image
- When option -o is given, all marked files get printed

Reported by Kris Siwiec

Fixes issue #84