Save and restore linked windows too

I sometimes link windows from a session to another, and currently after save and restore tmux-resurrect will duplicate the window instead of linking.

I tried to find out the best way to store the information in the resurrect file, but couldn't find one yet. The first idea was to store the linking information in window listing, but I read somewhere that linked windows are specific to a session and if one uses grouped sessions we might need different approach.

I am able to make a pull request if I find the best approach, but currently I'm not confident with the concepts.