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
