Look into switching from tkinter framework to Kivy

I wanted to start this with [Kivy](https://kivy.org/), but it apparently seemed to be broken on Ubuntu 17.10: https://github.com/kivy/kivy/issues/5453

But Kivy seems much better than tkinter for our specific case:
- It seems it has better support for animations
- It also is targeted towards touch interfaces (for possible swiping)
- Additionally works for Android