Add plakate navigation that works on the display

[christophfriedrich]

Swiping through the posters works fine on my laptop's touchscreen, but not on the infodisplay outside the Fachschaft office. This is probably not due to that touchscreen (I connected the screen to my laptop and then it worked) but due to the Raspberry Pi not realising the touch gestures.

On top of that, UI-wise it's not very obvious that one can swipe through the posters, so a navigation with prev and next buttons might be a better alternative that is even simpler to implement.