timeline.tt: NewStatusesButton and MoreStatusesButton are now implemented as proper classes. This makes it clear how components are connected to each other.
timeline.js: StatusContainer: add listenOnThresholdLevelChanged() method.
timeline.tt: NewStatusesButton: add _visibleUnackedCount() method.
timeline.tt: NewStatusesButton: now it shows only unacked counts of visible statuses.
timeline.tt: poller starts after the initial loading. If the poller starts earlier, it shows "New" count that goes to zero soon.
timeline.tt: add ThresholdLevelButton.