Update to sxiv-1.2.

Changes:
 * Added external key handler, called on keys prefixed with Ctrl-x
 * New keybinding { / } to change gamma (by András Mohari)
 * Support for slideshows, enabled with -S option & toggled with s
 * Added application icon (created by 0ion9)
 * Checkerboard background for alpha layer
 * Option -o only prints files marked with m key
 * Fixed rotation/flipping of multi-frame images (gifs)