None of the mouse mappings uses a keyboard modifier, making it possible to
access the most basic features by only using the mouse.

Next/previous image with left button depending on cursor position, middle
button for dragging, right button for switching to thumnail mode and wheel for
zooming.

Users can keep the old behaviour by simply not adapting the changes to the
buttons array in config.def.h to their config.h file.

No need to give cursor name. reset_cursor() is now the only place to decide
what cursor to fall back to.

This makes mouse panning more direct and faster.

- Functions warn() and die() replaced by GNU-like error(3) function
- Register cleanup() with atexit(3)
- Functions called by cleanup() are marked with CLEANUP and are not allowed to
  call exit(3)

- Key mappings +/- are now general commands
- Use JPG as thumbnail cache file format instead of PNG
- Fixes issue #161

- Only load the thumbnails that are currently visible in the window
- Unload thumbnails that are leaving the visible area
- Much less memory needed, but scrolling is now slower
- This also unintentionally fixes issue #86

- New option `-a`: Play animations at startup
- Ctrl-Space toggles animation for all GIF files
- Infinite loop for all animations

Issue described here:
https://bbs.archlinux.org/viewtopic.php?pid=1117294#p1117294

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

Fixes issue #84