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.

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)

This reverts commit f795273b65.

- 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

- Configurable key and mouse mappings in config.h
- Put event handling code from main.c into events.[ch]