The recent addition of "#{source_files}" allows us to avoid hardcoding
the file name and instead ask tmux itself for the very file it used to
create the session in the first place, i.e. "-f ./chat.tmux.conf".
Provide a hotkey to browser the manual in its own window.
After input, nicm (tmux upstream) added "-S" to tmux(1) such that
the "new-window" command (in combination with "-d") first looks
for the given window name and selects the window if it exists
instead of trying to create a window that already exists.
Given that this makes chat.tmux.conf idempotent, we can now also reload
it at runtime to refresh settings.
Klemens Nanni