quickSettings: Rename 'label' property to 'title'

We'll soon add a subtitle, so rename the label property to something
that is a bit more semantic.Add a warning when trying to set the old
'label' property.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2619>

js/ui/status/rfkill.js

@@ -93,7 +93,7 @@ const RfkillToggle = GObject.registerClass(
 class RfkillToggle extends QuickToggle {
     _init() {
         super._init({
-            label: _('Airplane Mode'),
+            title: _('Airplane Mode'),
             iconName: 'airplane-mode-symbolic',
         });