DmfsTask: introduce DmfsTaskFieldHandler interface + first handler classes

[rfc2822]

Part of #116. Depends on #338.

Analogous to: events synctools #76

synctools changes

• Create mapping/tasks/handler/DmfsTaskFieldHandler.kt interface (mirrors AndroidEventFieldHandler for events)
  • Core signature: fun process(from: ContentValues, to: Task)
  • Separate interface for property rows: DmfsTaskPropertyHandler (for alarms, categories, relations, …)
• Extract initial concrete handlers as proof-of-concept: UidHandler, TitleHandler, plus one property handler (e.g. AlarmsHandler)
• Refactor DmfsTaskProcessor to delegate to these handlers
• Add unit tests per handler class

DAVx5 changes

None needed.