Add GtkFlipper

No idea if it is useful to have, but I've had @YaLTeR, @chergert and 2 others I don't remember asking about rotating widgets in recent times. And whenevr I tell people to just write such a widget, they say it's too hard. Or they even use a drawing area with Cairo.

So now it's here, so I can link it when people ask again.

Or we can merge it.