xt/js/status_container: add tests on how cursor should move when the threshold level changes.
timeline.js: StatusContainer: countPerPage and maxPageNum params for loadStatus() class method are now mandatory. The values for there params when loadUnackedStatuses() is called are explicitly defined as class constants. This improves the clarity of constant declaration.
xt/js/status_container: add test for _ackStatuses(statuses, false)
timeline.js: StatusContainer: fix a bug in _adjustCursor(): jQuery.next(selector) returns empty set if the very next element does not match the selector. It won't search further.
timeline.js: StatusContainer: call window_adjuster() again at the end of the whole process to adjust window position appropriately.