popupMenu: Handle keypress if numlock is enabled

On Wayland, navigating menus with the keyboard would not open drop-down
menus when NumLock is enabled.

That's old issue (gnome-shell#550) that was not completely fixed with
commit 88556226 because the lock mask needs to be filtered out in
_onKeyPress() as well.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/550