Display embedded cover images

Summary:
introduce EmbeddedCoverageImageProvider to get access to embedded covers

store the fact that tracks have an embedded cover image in database

when importing files, check and store the presence of embedded cover

makes MusicAlbum and MusicAudioTrack use the embedded cover images

BUG: 380442

Test Plan: embedded covers are shown. albums without a cover but with tracks havong embedded cover are now showing a nice cover

Tags: #elisa

Differential Revision: https://phabricator.kde.org/D17163