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
When running the GUI, we generate a unix domain socket path for
the current process and start up a mux server for that path.
This allows `wezterm cli list` and `wezterm cli split-pane` to
work implicitly inside the GUI session.
When started in this way, the mux server is not persistent;
when the GUI process is terminated, all of its windows, tabs
and panes are terminated.
refs: https://github.com/wez/wezterm/issues/230
