...and implement it for GtkCssGadget and GtkCssCustomGadget.
This allows us to decide on a per-object basis if a custom
gadget needs a render node for content or not.
As GtkCssNode has the visibility concept, it makes sense to mirror it in
gadgets.
Do what visibility does in widgets: Hidden gadgets can't be drawn or
allocated and request a 0x0 size.
Note that just like widgets, gadget visibility must not be changed in
size request, allocate or draw handlers.
GtkWidget::child-visible has no equivalent yet, code will have to
emulate that manually.
A gadget is halfway between a widget and a CSS node. It's supposed to
provide the minimum convenicence around CSS nodes until we've figured
out how to integrate them with widgets.
