Adding the shared abstract Game class

We start by creating the Game.cs file that will contain shared code. Let's set this up by going through the following steps:

1. In the WhackABox project, create a new file called Game.cs in the root of the project.
2. Add the following code to the class:

using System;
using System.Linq;
using Urho;
using Urho.Shapes;

namespace WhackABox
{
    public abstract class Game : Application
    {
        protected Scene scene; 

        public Game(ApplicationOptions options) : base(options)
        {
        } 
    }
}

The Game inherits from Urho.Application, which will do most of the work regarding the game itself. We define a property called scene, of the Scene type. A Scene in Urho represents one screen of the game (we could have different scenes for different parts of a game or for a menu, for example). In this game, we will only be defining one scene, which will be initialized later. A scene maintains a hierarchy of nodes that compose it, and each...