Removed strange assortment of timeouts with different frequencies from TimerBreakController. We now have a single timeout to check the countdowns, and another timeout in ActivityMonitor to poll activity. Both timeouts run every second.
Connected shell unlock signal all the way through to a (placeholderish) "Break interrupted" notification for rest breaks.
Switched to new GLib.Thread API.
Share a single ActivityMonitor instance between break controllers. (This makes sense now, because it's turned into a much more complex object than originally planned).