Add Control.IsMouseCaptured, CaptureMouse, and ReleaseMouseCapture APIs

[cwensley]

This adds the ability to capture the mouse to a particular control/window until ReleaseMouseCapture is called. You can also tell if the mouse is captured during a mouse down/drag/mouse up operation with the IsMouseCaptured property.

New APIs:

class Control
{
  public bool IsMouseCaptured { get; }
  public bool CaptureMouse();
  public void ReleaseMouseCapture();
}