Grouped sessions are session started with the command
`tmux new-session -t <existing-session-name>`
Those sessions share the panes and windows with the existing, original
session.
We're making sure to identify and save grouped sessions.
On the other hand, we don't want to save panes and windows twice
The problem is that tmux list-window shows only the current pane layout
if a pane is maximized. This is a bug in tmux. In order to avoid this
bug we unzoom the window when saving and zoom in again after saving.
This implies that the Z flag is no longer set in list-windows, and so it
can't be used when restoring. Instead we use the Z flag of the panes
(which still have it) to restore the zoom.
