Release v2.0/going free (open source) software

Good news everyone,

I have reimplemented the game in Godot 3 (from Godot 2). This is basically a complete rewrite. It has cleaner code and some minor improvement. As such I have released it under the MPL2 and it is available on github: https://github.com/gapophustu/4D-Minesweeper

The previous version is still available as as the legacy branch on steam.