ChildWindow: added DialogResult property, user can set the DialogResult to true/false/null and the window will close, when close button pressed dialog result is false.
ChildWindow: can now show ChildWindow by using data binding on the WindowState property or by calling Show/Close in code.
ChildWindow: added Closed and Closing events. During the Closing event a user can set e.Cancel to true and the close will be cancelled and Closed event will not fire.