Introduce a Cache class

Summary:
This should help caching the data we tend to throw away out of the
Storage. This will speed up our tree generation and also allow to
provide a better service to features like contexts workday.

This is not connected to anything yet, will come in further commits.

Reviewers: Zanshin, bensi, dfaure, franckarrecot

Reviewed By: Zanshin, bensi, franckarrecot

Differential Revision: https://phabricator.kde.org/D5357