LocalCollection: fix "coincidence" naming

[rfc2822]

Currently, some implementations of the LocalCollection interface rely on the "coincidence" that LocalCollection has the same naming convention / method signatures as for instance the AndroidCalendar from ical4android. Example:

• LocalCollection : fun delete(): Boolean
• AndroidCalendar : fun delete(): Boolean

So a sub-class of AndroidCalendar implements LocalCollection "automatically".

This is not documented, error-prone (because if for instance the return value of delete in ical4android changes, DAVx⁵ compilation breaks) and bad design.

Possible solution: Different naming, for instance deleteCollection() in the interface, and then in LocalCalendar:

override deleteCollection() /* (from the interface) */ = delete() /* (from AndroidCalendar) */

etc.