DmfsTask: migrate all task field readers to individual handler classes #342

Part of #116. Depends on #341.

Analogous to: events synctools #79 + synctools #81 + synctools #106

synctools changes

Split the large DmfsTaskProcessor (230+ lines) into separate per-field handler classes in mapping/tasks/handler/:

Main task row fields

• UidHandler, TitleHandler (from DmfsTask: introduce DmfsTaskFieldHandler interface + first handler classes #341), SequenceHandler
• DescriptionHandler, LocationHandler, GeoHandler, ColorHandler, UrlHandler, OrganizerHandler

Status fields

• PriorityHandler, ClassificationHandler, StatusHandler, CompletedHandler, PercentCompleteHandler

Time fields (consider splitting like events #106)

• StartTimeHandler, DueHandler, DurationHandler

Recurrence

• RecurrenceFieldsHandler

Property row handlers

• AlarmsHandler, CategoriesHandler, CommentsHandler, RelationsHandler, UnknownPropertiesHandler

DmfsTaskProcessor itself becomes a thin orchestrator. Each handler class gets its own unit test.

DAVx5 changes

None needed.