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/system.js

@@ -87,7 +87,7 @@ const PowerToggle = GObject.registerClass({
         });
 
         this.set({
-            label: _('%d\u2009%%').format(this._proxy.Percentage),
+            title: _('%d\u2009%%').format(this._proxy.Percentage),
             fallback_icon_name: this._proxy.IconName,
             gicon,
         });