Refactor tasks (tasks.org/OpenTasks) to separate layers for storage + mapping

[rfc2822]

See also: https://github.com/bitfireAT/davx5/issues/670

We can use the new event architecture (at.bitfire.synctools.{icalendar,mapping.calendar,storage.calendar}) as a template.

Done when:

• Storage, mapping and iCalendar access are separated
• No separate data class (Task) required anymore
• Storage API is in at.bitfire.synctools.storage.tasks package
  • Collection (similar to calendar): task list
  • Are exceptions of recurring tasks supported? If yes, we need additional classes
    • Investigated by Find out supported features of OpenTasks and tasks.org #374 (see there for details)
  • Item: task, stored in Entity that represents rows in task provider
• Mapping API is in at.bitfire.synctools.mapping.tasks package
• iCalendar API is in at.bitfire.synctools.icalendar package
• All classes have proper tests

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Issues + PRs

• Like LocalCalendar/LocalEvent: provide all fields for DAVx5 #15

  • DmfsTaskList/DmfsTask: provide all fields for DAVx5 #138
  • LocalTaskList/LocalTask: Consume fields provided by synctools davx5-ose#1811

• Like Remove possibility to subclass AndroidEvent #20

  • Remove possibility to subclass DmfsTask #155
  • LocalTask: Don't subclass DmfsTask davx5-ose#1862

• Like Don't allow subclassing of AndroidCalendar anymore #23

  • Disallow subclassing of DmfsTaskList #160
  • LocalTaskList: Stop subclassing DmfsTaskList davx5-ose#1882

• Like Move logic from AndroidCalendar companion object to AndroidCalendarProvider #28

  • Move logic from DmfsTaskList companion object to DmfsTaskListProvider #170
  • DmfsTaskList refactoring davx5-ose#1934

• Like Introduce mapping classes #32

  • Introduce tasks mapping #174

• Like Event: extract EventReader/EventWriter #41

  • Extract task reading and writing from Task data class #176
  • [synctools] Tasks rewrite: Use reader/writer davx5-ose#1959

• Like refactor AndroidEvent; introduce AndroidEvent2 with Entity as data object #40

  • Refactor DmfsTask, introducing Entity as data object #182

• Like Read/write events as EventAndExceptions data object #45

• Like Add new Android event builder logic #65

• Like Event processor: create processor interface #76

• Like Migrate event sub-row builders #67

• Like More builders #71

• Like More processors #79

• Like Refactor remaining field processing into dedicated processors #81

• Like Workaround: don't UPDATE eventStatus to null #85

• Like Only generate recurrence fields/properties for recurring main events #89

• Like Ignore recurrence rules whose UNTIL is on or before DTSTART #95

• Like Split TimeFieldsProcessor #106

• Like Remove EventValidator #107