Old snprintf calls could have overflowed the buffers.

- 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

Without this some window managers (e.g. fvwm) will ignore the initial
window position and place it according to it's own rules.

- Draw onto pixmap as before, but use the same size for the pixmap as
  for the window, allocate new pixmap after configure requests
- Use XCopyArea() instead of XSetWindowBackgroundPixmap(), which now
  requires handling of Expose events

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