"Hardware-less" Photo booth usage support

[ClemensElflein]

I created this issue for all changes that need to be implemented in order to use the self-o-mat photo booth software without external hardware.

I think that using the self-o-mat software in such a minimal environment is not necessary, since an Arduino Nano (clone) only costs a few dollars and is all you need in order to gain full self-o-mat software. But I'm open for discussion.

At the moment I can think of the following features:

• On-Screen Countdown
  At the moment, the count down time is handled by the external hardware. The photo booth software can be triggered using a mouse click but it will trigger instantly and therefore is not suitable for real-life photo booth usage. The mouse click to trigger feature was only intended to help development.
• GPIO Button Support
  In order to trigger the photo booth without external hardware, it would be nice to allow triggering via GPIO
  GPIO pins for button #6
• Shut Down
  Currently, the external hardware can tell the self-o-mat software to write images to usb, shut itself and the Raspberry Pi down.
  Quit Program #4