Apps

This pull requests adds a new type of task that for lack of a better name I called "App". The idea is that only one app is allowed to be running at any given time. This should make access to shared resources like buttons, display and LEDs easier. There's an app switcher that ensures this and that all "background" apps are suspended. 

To demonstrate this I added a HomeScreen and a Flashlight app. The former is rather simplistic at the moment given that I haven't added display stuff yet, so I made the rgb led blink in different colours when it's running. The Flashlight app lights the leds in white but also allows the brightness to be changed via the up/down buttons. They're both rather simple, but that's kind of the idea.

All comments are very welcome!