sl0p: document the pane status bar protocol

Reference for programs wanting to drive the bar: the OSC 5577 verbs, escaping
rules, click reports, limits, rendering behaviour, persistence, and the security
model.

Both worked examples were run against a real session rather than written from
memory, which caught four bugs in my own samples:

- the Python reader kept a trailing buffer window, so a processed report was
  re-matched and every click fired twice
- it used tty.setraw, which also disables signals, leaving the sample unkillable
  with Ctrl-C
- it exited via traceback and left the bar behind; both samples now clear it on
  exit
- the shell reader echoed the report to the screen, and an `stty -echo` did not
  help because bash's `read -n` saves and restores terminal settings around each
  call. `read -s` is the fix.

Also verified and documented two behaviours I would otherwise have guessed at: a
BEL terminator works as well as ST, and the bar is not drawn when pane frames are
off (it lives in the frame row), reappearing when they return.

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