Factorize attribute handling into new AttributeStorage class.

Summary:
This fixes jobs sending unmodified attributes, potentially overwriting
changes made elsewhere.

Port Collection and Tag to use AttributeStorage.
Item will be for a separate commit.

Test Plan: All tests pass.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D19556