Make GstVideo/AudioInfo GLib.Opaque structures

Make GstVideo/AudioInfo GLib.Opaque structures

So that we generate a ABI compatible structure and make it working.
Also this is the way we do it for almost all other structures of this
kind.

I first tried to keep having a managed structure that properly matches
the C structure ABI generating in the gtk-sharp generator Explicit structure
for the union as we do with non managed structures but this is not
possible and leads to the following assertion in mono:

    Type Gst.Video.VideoInfo/__ABI which has an [ExplicitLayout] attribute cannot have a reference field at the same offset as another field.

This sensibly changes the API but I bet noone will even notice.

https://bugzilla.gnome.org/show_bug.cgi?id=788133