qml: if title is empty use "Unknown Title" in playlist delegate (!6427) · Merge requests · VideoLAN / VLC

Currently, if title is empty:

In ArtworkInfoWidget, length gets centered. We probably don't want to show "Unknown X" there to not occupy space, centering seems to be a good compromise.
In PlaylistDelegate, if artist is empty it prints "Unknown Artist", and the field for the title remains empty. This is not coherent, and it does not look good.

Request review @chub.

qml: if title is empty use "Unknown Title" in playlist delegate