Adds a window to application.
This call can only happen after the application has started; typically, you should add new application windows in response to the emission of the activate signal.
This call is equivalent to setting the application property of window to application.
Normally, the connection between the application and the window will remain until the window is destroyed, but you can explicitly remove it with Application.removeWindow.
GTK+ will keep the application running as long as it has any windows.