The node declaration has the same functionality as
gtk_css_node_declaration_add_to_widget_path(). So instead of using that
function on a path, you can use the original path and the declaration in
a matcher.
GtkCssNodeDeclaration is a new struct with copy-on-write semantics.
It encapsulated the properties used to define a node in the CSS tree.
The idea is to use it in various places for caching, in particular as
key in hash tables.
