WPF - IWpfWebBrowser implement IInputElement

Simplify setting Focus from View model as now possible to call Keyboard.Focus(IWpfWebBrowser);

Resolves #3546

Author: amaitland

CefSharp.Wpf.Example/ViewModels/BrowserTabViewModel.cs

@@ -207,7 +207,7 @@ private void Go()
         {
             Address = AddressEditable;
 
-            Keyboard.Focus((IInputElement)WebBrowser);
+            Keyboard.Focus(WebBrowser);
         }
 
         public void LoadCustomRequestExample()

CefSharp.Wpf/IWpfWebBrowser.cs

@@ -3,6 +3,7 @@
 // Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.
 
 using System;
+using System.Windows;
 using System.Windows.Input;
 using System.Windows.Threading;
 using CefSharp.Enums;
@@ -15,7 +16,7 @@ namespace CefSharp.Wpf
     /// and properties the <see cref="ChromiumWebBrowser" /> exposes.
     /// </summary>
     /// <seealso cref="CefSharp.IWebBrowser" />
-    public interface IWpfWebBrowser : IWebBrowser
+    public interface IWpfWebBrowser : IWebBrowser, IInputElement
     {
         /// <summary>
         /// Navigates to the previous page in the browser history. Will automatically be enabled/disabled depending on the