Disable track number for songs that don't have it

Summary:
This patch hides the track number for tracks that don't have it in the album, playlist, track and metadata views.

Reviewers: Elisa, mgallien

Reviewed By: Elisa, mgallien

Subscribers: mgallien, astippich

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D10000