Wpf.HwndHost - Notify WPF when Browser has focus

Directly clicking the mouse in the browser and CEF will recieve focus, we need to let WPF know that our HWND host has focus in this scenario.

We need to set the Logical focus, not the actual focus.
Something like the following might work.
```c#
var focusScope = FocusManager.GetFocusScope(this);
if (FocusManager.GetFocusedElement(focusScope) != this)
{
FocusManager.SetFocusedElement(focusScope, this);
}
```

https://referencesource.microsoft.com/#PresentationCore/Core/CSharp/System/Windows/UIElement.cs,2647

When the browser directly gets focus `IFocusHandler.OnGotFocus` should be called.

Follow up to cefsharp/CefSharp.Wpf.HwndHost#5