Kivy crash course 4: Kivy language HD

This time, I convert all the behaviour of our program straight to kv language...hopefully explaining how this makes it clearer and more flexible on the way! Download links: - Initial code download: https://github.com/inclement/kivycrashcourse/blob/master/video4-kivy_language/before.py - Final code download: https://github.com/inclement/kivycrashcourse/blob/master/video4-kivy_language/after.py - Final kv file download: https://github.com/inclement/kivycrashcourse/blob/master/video4-kivy_language/tutorial.kv