Control-click -> new window

This is releted to a feature in WebDriver that I want to support. When clicking a link with Ctrl or Shft pressed, the Url should open in a new window. We already have Windows and frames and support for targets built in, so extending the AnchorElement.Click() to take the key state into account shouldn't be too hard.

I've also added an event that fires every time a new browser window is opened. Makes working with these scenarios a lot easier.

Can someone please review? I can merge myself, but I feel that we should try to maintain a four-eyes policy for non-trivial changes.