lockScreen: Extend modal grab to entire uiGroup

Main.pushModal() used to ensure a compositor grab, but didn't
take an input grab. That changed when porting to ClutterGrab,
so by limiting the grab to the shield/dialog, it is no longer
possible to access top bar menus when the screen is locked.

To fix that, take the grab on the whole uiGroup so that the top
bar is included.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5470

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2298>

js/ui/unlockDialog.js

@@ -873,7 +873,7 @@ var UnlockDialog = GObject.registerClass({
             timestamp,
             actionMode: Shell.ActionMode.UNLOCK_SCREEN,
         };
-        let grab = Main.pushModal(this, modalParams);
+        let grab = Main.pushModal(Main.uiGroup, modalParams);
         if (grab.get_seat_state() !== Clutter.GrabState.ALL) {
             Main.popModal(grab);
             return false;