Same vein as 8931afba5cee07ab12990f06c2ff34d6f8426b19; the auth
window could sometimes get stuck until an input event was sent
to it.
Wire up a mux event so that the window can close itself.
in the same vein as d657721163
this commit introduces more assertive signalling from the remote
mux when a pane is closed so that the client can update.
As part of reducing the amount of regularly scheduled stuff wezterm
does in the background, this commit restructures how an empty mux
is detected; now when the mux prunes dead windows it will emit
an Empty event.
The Activity type will now schedule a prune when it is dropped,
which will clean up and trigger the Empty event.
refs: https://github.com/wez/wezterm/issues/770
