Port to 0.9.12

[pintergreg]

Hey @kekekeks ,

I'm started to work with Avalonia and I found this project a very good start my goal application, but I could not run it with my config.
Thus, I ported it to the current Avalonia (and .NET Core 3.0), here is my repo: https://github.com/pintergreg/BattleCity

I needed to modify the App.xaml.cs and the DataContext in MainWindow.xaml, though the latter does not feel the best way, but I'm still not familiar with Avalonia. I also removed the Annotations.cs as I don't understand its role.
Now it works, although there is an error after closing it if I let it run for a while.

You did not set a license, so I do not know that how I can use your code, but you can use my modifications freely.

Regars,