Move "selected" flag from command to pane.
Add a command for mirroring panes.

This creates a new pane that's linked to the selected pane's command,
so that diff/graph/etc. can be set on it while still viewing the
command's output separately, without having to run the command twice.

This has a significant number of edge cases (updates, config files,
per-pane pausing) that haven't been worked out yet.