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 the title, icon, OSC 7 and SetUserVars sequences are processed,
notify the embedding application.
The gui layer uses this to trigger a titlebar update.
refs: #647
