Option to allow overlay windows to be an external window by default

[Armored-Dragon]

Description

It would be a good convenience factor to allow any overlay window created on screen to be able to become an external window. An option would probably be best located in the window decorations.
I may be able to include this in the #886 pull but I think this may require some C++ magic.

[ksuprynowicz]

I think it's a really good idea. We would need to check if camera views render in external window for spectator camera though.