multi-attach: say who else is watching, and where you are

A small tag in each client's own top-right corner when several
terminals share the session: the viewer count, and on a view that is
a crop being panned by somebody else's cursor, the crop's offset into
the shared screen ("2 clients +12+4") -- which is exactly the moment
a screen moving on its own needs explaining. Stamped per client after
projection, because every terminal has its own count of others and
its own offset; the shared status bar is composed once and can be
cropped out of a panned view anyway. attach_indicator false hides
it.

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