Add recurrence support to the Task Domain

Summary:
This is an adapted and simplified version from Franck's work toward
recurrence support. This version has no linkage between start date and
recurrence. It means that setting a recurrence but no dates on the task
will simply do nothing special.

Reviewers: dfaure, bensi, franckarrecot

Differential Revision: https://phabricator.kde.org/D7266