Draft: fix undefined errors

Errors regarding item focus, after double-clicking an artist, a music genre or a service, respectively:

qml message qrc:/medialibrary/MusicArtistsDisplay.qml:75 ReferenceError: stackView is not defined
qml message qrc:/medialibrary/MusicGenresDisplay.qml:75 ReferenceError: stackView is not defined
qml message qrc:/network/ServicesSources.qml:83 ReferenceError: root is not defined

Errors regarding double-clicking a music album:

qml message qrc:/widgets/KeyNavigableTableView.qml:338 Unable to assign [undefined] to QString
qml message qrc:/medialibrary/MusicAlbumsGridExpandDelegate.qml:146 ReferenceError: expand_cover_id is not defined
qml message qrc:/widgets/KeyNavigableTableView.qml:338 Unable to assign [undefined] to QString