PanelMenu.Button listens to key-press-events on the corresponding
menu to move focus to neighboring buttons on left/right arrow. In
order to allow keynav from the category list on the left to the
application list on the right and vice-versa, we may need to suppress
the parent behavior depending on the currently focused actor, so
overwrite the signal handler accordingly.
https://bugzilla.gnome.org/show_bug.cgi?id=693074