Add properties to the IWebBrowser interface

* Add three properties to the XWebBrowser interface to
  enable features used on the browsers in Chorus.
* Add WinFormsBrowserAdapter handling of those properties.
* Add todo comments where these features are as yet unimplemented
  in the GeckoFxWebBrowserAdapter
* Fix DocumentText content loading for Geckofx14
Enable the WebShortcutsEnabled property on XWebBrowser

* Pass the property on to the adapters
* Implement in the Winforms browser
* Add comment and no-op implementation for Gecko browser
Merge pull request #39 from jasonleenaylor/EnhanceXWebBrowser

Add properties to the IWebBrowser interface