Port Item to AttributeStorage, so ItemModifyJob doesn't send unmodified attributes.

Summary:
This required moving Attributes into ItemChangeLog, which now hosts
the AttributeStorage instance for the item. Since the separation of
ItemChangeLog was made to reduce memory consumption, I hope most items
don't have any attribute, otherwise this is a bad choice.

Test Plan: unit tests pass

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D19632