Lift setting() onto the Panel base too

Matches the BarWidget extraction in the previous commit. The weather
panel was the only panel reimplementing the helper inline; future
panels with inline settings get it for free.

shell/Ui/Panel.qml

@@ -21,6 +21,13 @@ Item {
   function close() { panelController.hide() }
   function toggle() { opened ? close() : open() }
 
+  // Read a single value from this panel's inline shell.json entry, with a
+  // fallback for missing/null values. Matches BarWidget.setting().
+  function setting(name, fallback) {
+    var value = settings ? settings[name] : undefined
+    return value === undefined || value === null ? fallback : value
+  }
+
   PanelController {
     id: panelController
   }