Avoid duplicated tasks in Workday page

Summary:
We now use the cache in order to check if a task supposed to be in the
workday has a parent in the workday. In that case we don't need it as
top level since it'll be brought up again by its parent.

Reviewers: dfaure, bensi, franckarrecot

Reviewed By: bensi

Differential Revision: https://phabricator.kde.org/D6922