Clean up events

[suluke]

We introduced the datepicker-change-incomplete event, which is only used by top-bar so far. This overlaps with the datepicker-first-date-selected event.

The semantics of the events should be specified, i.e.:

1. datepicker-change indicates that the picker has either all or none of the required dates set.
2. datepicker-first-date-selected is only triggered in date_range_ mode (not singledate) and indicates that the picker is currently in a whacky/illegal state where the user is expected to select the second state.

The triggerHasChanged method should take care of triggering all of the above events.
The invalidate method and checkSelectionValid methods need to be checked if there is code that is not required