Don't push private APIs in the public 'interface'

Summary:
The current interface mixes really public stuff
and stuff required only for MK4 or the Dummy storages.
This makes it far harder to understand 'who' does 'what'.

This commit drops a part of that legacy.

Reviewers: mlaurent, knauss

Reviewed By: mlaurent

Subscribers: KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D12451