panelMenu: Fix keynav of menu-less buttons

The left/right navigation between top bar buttons is usually
handled by a key-press handler on the button's menu.

However when a DummyMenu is used, the button itself serves as
fake menu actor and will get grabbed when "opening" the menu.
Due to that grab, the event is not propagated to the stage,
and regular keynav does not work.

To avoid the focus getting stuck in that case, add an explicit
key-press handler that bypasses the grab.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2734>
(cherry picked from commit ecb274ce)