drive-menu: Fix indicator visibility

Commit 519269be9d made the check for network mounts non-blocking, and
we now update the indicator's visibility before a newly-added network
mount is hidden.

Address this by monitoring the item itself for visibility changes.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/176>

extensions/drive-menu/extension.js

@@ -189,6 +189,8 @@ class DriveMenu extends PanelMenu.Button {
         let item = new MountMenuItem(mount);
         this._mounts.unshift(item);
         this.menu.addMenuItem(item, 0);
+
+        item.connect('notify::visible', () => this._updateMenuVisibility());
     }
 
     _removeMount(mount) {