do not forget the album cover when using embedded cover images

Summary:
Use embedded cover images when inserting an album in database allowing tracks and albums to use the right cover image

Currently require a reimport of database. Will fix that in a later revision.

CCBUG: 393486
BUG: 406482

Test Plan: Auto tests are OK. Attached test files show an embedded cover image.

Reviewers: ngraham

Reviewed By: ngraham

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D20879