Access session group id

[DavidPL1]

When a new session is created via the tmux new-session -t "name" command and a session with that name already exists, a new group will be created with both the original and the new session as members or the new session will be added to the already existing group. This is useful when you want to attach a window within the original session without actually changing the selected window of the original session. Let this new session be called clone session.

Now my approach of getting the name of the 'clone' session would be to filter sessions by group id, but group ids are currently not featured in the server.list_sessions() call, thus it would be nice if this could be added.

Building on that, a call to 'clone' a session with the option of implicit renaming could be implemented.