Small fixes for all-day events

[timonegk]

This PR introduces some fixes for all-day events:

• the TimeFrame filter should not filter all-day events because if it does, there is no way to filter for business hours but keep the all-day events.
• for all-day events, only the dates should be compared, not the time. The reason for this change is that all-day events in zep start and end at 00:00 local time zone, whereas all-day events in google start and end at 00:00 UTC. Therefore, the event is always shown as "changed".