Solves several issues, generally through delivering signals before filesystem events, preventing situations where an overwhelming amount of events makes it impossible to quit.
Does _not_ solve the problem of a queue full of lower-priority events not accepting an urgent message, but that's a rarer issue that's more complicated to overcome.
Changes the Filterer trait: adds Priority to `check_event()`
Makes some events unfilterable (Urgent priority): SIGINT, SIGTERM, and CTRL_C to the main process. These still need to be handled by `on_action` to do anything, but cannot be stopped before reaching that.
Also change the concept of a completion handler to instead sending a
synthetic "process completed" event down the same path as usual.
That makes handling completion the job of the action handler, but also
means it's immediately possible to launch a process or do an action in
response to the process completing. Win win!
