In the next commit, we will turn PanelMenu.ButtonBox into a
St.Widget subclass. As a domino effect, PanelMenu.Button will
become one too, and so will Panel.AppMenuButton.
When that happens, the current show() and hide() functions in
Panel.AppMenuButton will clash with Clutter.Actor's ones.
To avoid that, rename these functions to fadeIn() and fadeOut()
and avoid a name clash.