After #7164, entities in the ActorViewer are loaded through the
preview filter, which disables shadows by design. As a result, the
shadow toggle no longer has any visible effect.
Adds a dedicated viewer.xml template so the ActorViewer no longer
inherits the preview filter's shadow behaviour, keeping preview.xml
untouched for the entities that legitimately need shadows off.
Refs #7164
Reported by: Vladislav