Move per-window notify-on-status-change out of the Session Status
toolbelt tool into a new iTermNotifyOnStatusChangeController so it works
regardless of toolbelt visibility, and add a per-session scope on top of
it. One-shot semantics are preserved: a watched session changing status
shows a single alert and disarms. A change that satisfies both a session
watch and its window watch consumes both but alerts once.

- ToolStatus bell now toggles the centralized state for the selected
  session, or the window when nothing is selected, and shows a per-row
  bell indicator for armed sessions.
- The Window menu routes through the controller by terminal guid.
- The Cockpit gains a bell toolbar item that arms the selected window or
  session, with an accent-colored bell shown at the leading edge of
  armed rows.
- The Cockpit panel uses the utility-window style so it no longer dims
  distractingly when it loses key status.