Add starter project for chapter 4

Chapter04/Start/MyMediaCollection/ViewModels/RelayCommand.cs

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+using System;
+using System.Windows.Input;
+
+namespace MyMediaCollection.ViewModels
+{
+    public class RelayCommand : ICommand
+    {
+        private readonly Action action;
+        private readonly Func<bool> canExecute;
+
+        public RelayCommand(Action action)
+            : this(action, null)
+        {
+        }
+
+        public RelayCommand(Action action, Func<bool> canExecute)
+        {
+            if (action == null)
+                throw new ArgumentNullException(nameof(action));
+
+            this.action = action;
+            this.canExecute = canExecute;
+        }
+
+        public bool CanExecute(object parameter) => canExecute == null || canExecute();
+
+        public void Execute(object parameter) => action();
+
+        public event EventHandler CanExecuteChanged;
+
+        public void RaiseCanExecuteChanged() => CanExecuteChanged?.Invoke(this, EventArgs.Empty);
+    }
+}