fix(tray): fix visibility of Passive items

`show_all` call from `Tray::update` attempts to walk the widget tree and
make every widget visible. Since we control individual tray item
visibility based on `Status` SNI property, we don't want that to happen.

Modify `Tray::update` to control the visibility of a whole tray module
only and ensure that the children of `Item` are still visible when
necessary.

src/modules/sni/tray.cpp

@@ -35,11 +35,8 @@ void Tray::onRemove(std::unique_ptr<Item>& item) {
 }
 
 auto Tray::update() -> void {
-  if (box_.get_children().empty()) {
-    box_.hide();
-  } else {
-    box_.show_all();
-  }
+  // Show tray only when items are availale
+  box_.set_visible(!box_.get_children().empty());
   // Call parent update
   AModule::update();
 }