Map each synced calendar to its own collection

When syncing Google Calendar or iCloud Calendar, let each individual calendar map automatically to its own collection. Today synced events arrive as read-only items that don't belong to any collection, so they can't be organised alongside the rest of a list. Note: this needs synced calendar events to become full entries that can carry a collection - the same prerequisite as being able to tick them off (https://feedback.bullet.to/p/ability-to-tick-calendar-events). Related: https://feedback.bullet.to/p/type-of-entry-for-integrated-calendar