Add support for tmux 2.3's pane status text

Since version 2.3 tmux is able to assign a title to a pane which can be shown using the pane-border-status option. Additionally the option pane-border-format allows to set that title. However, this option applies to all panes and if you want to have per-pane specific title you have to use escape sequences, at least I'm not aware of a more elegant solution. Using that, it is indeed possible to use this functionality with the current version of tmuxp, but the yaml file doesn't look very pretty:

          - shell_command:
              - printf '\033]2;%s\033\\' 'pane title'
              - actual_command

It would be nice to add a dedicated attribute for that, something like title or status that essentially does the ugly printf above.

Start by tracing how tmuxp reads YAML pane definitions and handles shell_command entries when creating panes. Compare that path with tmux 2.3's per-pane title escape sequence behavior. Done means a dedicated per-pane title or status attribute works without requiring users to write the printf sequence themselves.