Benjamin Otte authored Dec 23, 2015 and Cosimo Cecchi committed Dec 29, 2015

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.