Implement Focused property

Issue: `ChromiumWebBrowser.Focused` is always `false`
Cause : cef does not implement focused correctly (seems related to https://code.google.com/p/chromiumembedded/issues/detail?id=563)
Fix : Manually implement Focused
How to test: 
- In the Windows Form sample, replace the Go Button event handler by `MessageBox.Show(((Control)Browser).Focused.ToString())`. 
- run
- give the focus the the browser
- click Go
- result : false
- apply fix
- tests again 
- **result : true**
- click the address bar (to remove focus)
- click Go
- result : false

I think this should fix #570. However if it does not, then this fix is still relevant to any code that relies on the `Focused` property. I did not test the WPF side of this at all.

Background: We've been running with this fix for a while now (before we started contributing to this project). I recalled this fix while reading #570 so here is the PR (I'll be able to remove this from our custom ChromiumWebBrowser derived class too).

Can we move this class into `CefSharp.WinForms`? it's unlikely to be used in any of the other projects.