Bug 1083820 - [Calendar] update MonthsDay view to use DayObserver r=gaye

294 additions and **2,208 deletions**!!!
just realized that we should always put "all day" events at the top, even if we have other events that starts at midnight. will update this logic.

in fact I even think that we should probably update the day observer logic so the records are already split into "events" and "all day" events. it will avoid duplication and simplify things.