Being able to capture part of the screen (Windows, UIItems, etc.)

For approval tests to be more concise, you need to get screenshot of certain parts (UIItem) of the application. That's where the screenshot of the whole desktop won't be useful. I have created an overload where you can specify bounds for the screenshot.

Another change is making the screencapture class internal, as the interface to that is through the Desktop object.

Also note that I have taken a different approach to taking the screenshot so it relies only on managed code. Happy to send you another pull request to get rid of the unmanaged codes in ScreenCapture class.