gadget: Handle visibility

gadget: Handle visibility

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.