Tray::update() already had logic to hide the module when all child items were passive, but it was only re-run on item add/remove — never on a status change. When the last visible item transitioned to Passive, the item hid itself but the now-empty module remained visible.
Connect Tray to each item's Gtk::EventBox signal_show/signal_hide so update() runs on every visibility transition, and simplify update() to check child->get_visible() directly instead of inspecting the `passive` CSS class. The Tray-level `show-passive-items` read becomes redundant since Item already honours it when deciding its own visibility; remove it along with the now-unused Tray::show_passive_ member.
Fixes: #3721
