Avoid duplicated tasks in Context pages

Summary:
We now use the cache in order to check if a task tagged with the current
context also has a parent tagged in that same context. 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/D6920