DmfsTask: finish new storage-level DmfsTask API

[rfc2822]

Part of #116.

Analogous to: events synctools #45

Complete WIP PR #182 which introduces storage/tasks/DmfsTask.kt with Android Entity as the storage data structure.

synctools changes

• Finalise storage/tasks/DmfsTask.kt: holds Entity (main task row + property sub-rows for alarms, categories, relations, …)
• Update DmfsTaskList.findTasks() / getTask() to return the new storage.tasks.DmfsTask
• Add CRUD methods on DmfsTaskList for the new DmfsTask type (analogous to AndroidRecurringCalendar for events)
• The legacy ical4android/DmfsTask.kt is already deprecated in PR Refactor DmfsTask, introducing Entity as data object #182 (kept temporarily)

DAVx5 changes (separate PR)

• LocalTask.kt: wrap the new storage/tasks/DmfsTask instead of ical4android/DmfsTask
• LocalTaskList.kt: adapt to the updated DmfsTaskList API