Convert app module to Kotlin

[depau]

I did some minor testing, I found some issues that I haven't solved but I'm pretty sure are not related to the port, rather to recent Android changes:

• MainActivity.onItemClick(): File.mkdirs() is not creating any directories even after granting the permission
• MainActivity.onStart(): It appears that we're not allowed any more to start services when the app is not in focus (i.e. you launch the app from Android Studio with the phone's screen off)

Other than that I did some additional kotlinization, converted a bunch of stuff to lateinit or vals to improve null safety.

[magnusja]

ahh man Kotlin ist just so much nicer :D