Add support for background events

Add support for background events. We need intended use zones reserved for specific events types and this has to appear on the calendar and must not collide with effective appointments.

Modification is only on the method `agendaSegsCollide ` which determines if there are collisions and it is neutral for existing API. Event hash now support the key `isBackground`.

Example usage:

```
events = {
	title: 'Intended use zone',
	start: new Date(),
	end: new Date(),
	isBackground: true,
}
```