Add MonoGame support

[hadashiA]

Add extensions for MonoGame.

public class Game1 : Game
{
    public Game1()
    {
        var observableSystemComponent = new ObservableSystemComponent(this);
        Components.Add(observableSystemComponent);
    }
}

ObservableSystemComponent configure the following:

• Setup TimeProvider and FrameProvider.
  • Time based operations are replaced with Game.Update(GameTime).
  • Frame based operations are replaced with Game.Update(GameTime).
• Set UnhandledExceptionHandler. By default, the unhandled exception handler simply flows to System.Diagnostics.Trace.
  • If we want to change this, do the following:

    • new ObservableSystemComponent(this, ex => Console.WriteLine($"R3 UnhandledException: {ex}");

R3Extensions.MonoGame provides these providers.

• MonoGameTimeProvider
• MonoGameFrameProvider

And provides these custom operators.

// Observe the current GameTime value.
public static Observable<GameTime> GameTime(this Observable<Unit> source)

// observe the current GameTime and the value of the source observable.
public static Observable<(GameTime GameTime, T Item)> GameTime<T>(this Observable<T> source)

[hadashiA]

I considered supporting GameComponent.AddTo, but decided against it.
There is no way to intercept in GameComponent.Dispose in an extension method. And GameComponent.Dispose seems to be called only in Game.Dispose by default.