Optional Content Groups (OCGs aka layers): initial support

I'm currently writing some code for a client to create and manipulate optional content groups (OCGs aka layers) on the PD-layer. The goal is to do overlays in a way that the various combined pages each are on a separate layer that can be switched on and off. Please note that this is only a start and, for example, does not include conditional rendering of layers. I'll submit a patch for review in a couple of days. I've got the necessary PD layer code already and now have to upgrade the Overlay utility.