loginManager: Return objects instead of multiple booleans

Multiple booleans - both in arguments and return values - are almost
always problematic API, because people have to memorize (or more likely
look up) the meaning of each position.

Instead, return a JS object so each value has a name attached to it.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2370>

js/ui/endSessionDialog.js

@@ -305,7 +305,7 @@ class EndSessionDialog extends ModalDialog.ModalDialog {
     }
 
     async _getCanRebootToBootLoaderMenu() {
-        const [canRebootToBootLoaderMenu] = await this._loginManager.canRebootToBootLoaderMenu();
+        const {canRebootToBootLoaderMenu} = await this._loginManager.canRebootToBootLoaderMenu();
         this._canRebootToBootLoaderMenu = canRebootToBootLoaderMenu;
     }