Restructure project

* use modules instead of source sets
* Kotlin JVM instead of multiplatform (for now) to make things simpler
* Warning: GameCell doesn't use mutableState anymore, this needs to be
           fixed for the UI to work!

android/src/main/res/mipmap-anydpi-v26/ic_launcher.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="utf-8"?>
+<adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
+    <background android:drawable="@color/ic_launcher_background"/>
+    <foreground android:drawable="@drawable/ic_launcher_foreground"/>
+</adaptive-icon>