Port to Xamarin

[ekolis]

Might require porting to .NET Core first (#31), but this would make the game truly cross platform, and open it up to a better UI framework than Windows Forms.

[ekolis]

I always wanted to make an Android client - well, Xamarin works pretty well with Android, in addition to Windows!

[ekolis]

Hmm. Xamarin doesn't run on Linux, but Avalonia supposedly does...