Control slideshow using physical buttons

It is possible use physical buttons to show next or previous image as well as to start/pause the automatic slideshow.

What pins are used can by configured in /data/photoframe/conf/photoframe.conf:

GPIO_PIN_NEXT= # GPIO to show next file
GPIO_PIN_PREVIOUS=-1 # GPIO to show previous file
GPIO_PIN_PLAY=-1 # GPIO to start/pause rotating images automatically
GPIO_ACTION_VALUE=0 # value to identify action, for an pullup the value should be 0, for pulldown 1