Use ICommand from System.Windows.Input.

Perspex.Controls/Button.cs

@@ -7,6 +7,7 @@
 namespace Perspex.Controls
 {
     using System;
+    using System.Windows.Input;
     using Perspex.Input;
     using Perspex.Interactivity;
 

Perspex.Input/ICommand.cs

@@ -1,19 +0,0 @@
-// -----------------------------------------------------------------------
-// <copyright file="ICommand.cs" company="Steven Kirk">
-// Copyright 2015 MIT Licence. See licence.md for more information.
-// </copyright>
-// -----------------------------------------------------------------------
-
-namespace Perspex.Input
-{
-    using System;
-
-    public interface ICommand
-    {
-        event EventHandler CanExecuteChanged;
-
-        bool CanExecute(object parameter);
-
-        void Execute(object parameter);
-    }
-}

Perspex.Input/Perspex.Input.csproj

@@ -60,7 +60,6 @@
     <Compile Include="FocusManager.cs" />
     <Compile Include="FocusNavigationDirection.cs" />
     <Compile Include="ICloseable.cs" />
-    <Compile Include="ICommand.cs" />
     <Compile Include="IFocusManager.cs" />
     <Compile Include="IFocusScope.cs" />
     <Compile Include="IInputDevice.cs" />