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
   }

shell/plugins/panels/weather.qml

@@ -65,11 +65,6 @@ Panel {
   readonly property string reportWind:      current ? (useImperial ? (current.windspeedMiles + " mph") : (current.windspeedKmph + " km/h")) : ""
   readonly property string reportHumidity:  current ? (current.humidity + "%") : ""
 
-  function setting(name, fallback) {
-    var v = settings ? settings[name] : undefined
-    return v === undefined || v === null ? fallback : v
-  }
-
   function refresh() {
     if (!forecastProc.running) forecastProc.running = true
     if (!locationProc.running) locationProc.running = true