pipe loading of album data through proxy model

Summary: pipe loading of album data through proxy model so that qml does not have to know about the album model, similar to all other proxy models

Test Plan: album data is loaded correctly from all albums view and single artist view

Reviewers: Elisa, mgallien

Reviewed By: Elisa, mgallien

Subscribers: mgallien

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D10643