DM Screen: hidden-characters list can miss older hidden NPCs

The DM Screen's Reveal list reads the 50 most recently updated entities of each NPC/creature type and keeps the hidden ones, then shows at most 8. In a campaign with many NPCs of one type, an older hidden one may never appear, and the panel can say nothing is hidden. Query hidden entities directly (is_private) and show a link to the Characters page when there are more than fit.