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/bluetooth.js

@@ -148,7 +148,7 @@ const BtClient = GObject.registerClass({
 const BluetoothToggle = GObject.registerClass(
 class BluetoothToggle extends QuickToggle {
     _init(client) {
-        super._init({label: _('Bluetooth')});
+        super._init({title: _('Bluetooth')});
 
         this._client = client;