GlowWindow closing fix

I'm currently working on an application where a user needs to login. After the login, a user can open several windows. When a user logs off, all these child windows need to be closed. This is done using `Application.Current.Windows`. However, all the GlowWindows are also part of this list and are closed, causing errors on the following GlowWindow.Update() call.

This can be fixed by only allowing a GlowWindow to be closed when its owner has been closed.