- Buffer for window content is bigger than the window, minimizes artifacts
  when window is resized
- Back to using XSetWindowBackgroundPixmap() instead of XCopyArea(),
  no need to handle exposure events; X server can show gray background directly
  after resize event before sxiv redraws the window contents

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

Fixes issue #143

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

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

Reported by Kris Siwiec