TODO
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 - load images alphabeticaly
 - if we press 'n' or 'p' a long time then don't load more than one image
   after releasing the key (other keyhandler?)
 - key 'f' for fullscreen
 - key 's' for toggling autoscale
 - key 'u': stretch the image on autoscale or not (unproportional scaling)?
 - display "Loading..." when loading a file
 - scaling with: mouse wheel or alt+left key (scale +/- 10%)
 - scaling with shift: scale +/- 1%
 - scaling with ctrl: scale +/- 100%
 - left button: move the image (with shift slower and ctrl faster)
 - middle button: revert
 - draw the filename, resolution and scale (toggle with 'i')
 - draw a small rectangle which shows the full image and a smaller rectangle
   which shows the current part (enable dragging this small rectangle)
 - preload next and previous file in background (pthreads?)
 - '.' and ',' for next/previous picture
 - '-' and '+' for zooming in/out
 - key 'h' for showing help
