Use `isAllDay` instead of `isSingleDayAllDayEvent` to decide top bar placement

[jonasc]

What is it?

• [ ] Bugfix
• [x] Feature
• [ ] Codebase improvement

Description of the changes in your PR

• Change shouldAddEventOnTopBar to not treat multi-day all-day events differently from single-day all-day events
• In my opinion, it makes much more sense to show multi-day all-day events always in the top bar (like single-day all-day events), independent of this config setting.

Fixes the following issue(s)

• Fixes #82

Acknowledgement

• [x] I read the contribution guidelines.