Add support for yearly recurrence.

Summary: This is useful for end-of-accounting-year tasks for instance :-)

Test Plan:
Created a task, set recurrence to yearly, checked it, the start date
and due date moved forward one year.

Reviewers: ervin, franckarrecot

Reviewed By: ervin

Subscribers: Zanshin

Differential Revision: https://phabricator.kde.org/D19810