[caldav] Ensure all required notebook incidences are loaded

Ensure that the client will be able to find all notebook incidences
when required.
[caldav] Change dtEnd correctly for all-day events and undo the changes on upsync

mkcal+calendar expect the dtStart/End to have times for all-day events,
so add these when importing all-day events, then remove the times when
exporting back to the server.
[caldav] Fix dtStart, dtEnd and allDay property updates

Copy all-day property after dtStart/End have been copied, as changes
to these will change the all-day value.
[caldav] Set exclusive DTEND when exporting events to the server

iCalendar format states DTEND must be exclusive. This must be set
else multi-day all-day events will not have the correct dates.
For single-day all-day events this wasn't a problem previously as
the dtEnd was not set at all, but it should be set anyway in this
case to be consistent.
Merge pull request #19 from nemomobile/all-day-events

Fixes for all-day events