New API for visibility in both CanvasItem and Spatial

visible (property) - access set_visible(bool) is_visible()
is_visible_in_tree() - true when visible and parents visible
show() hide() - for convenience

scene/2d/navigation_polygon.cpp

@@ -459,7 +459,7 @@ void NavigationPolygonInstance::_navpoly_changed() {
 
 String NavigationPolygonInstance::get_configuration_warning() const {
 
-	if (!is_visible() || !is_inside_tree())
+	if (!is_visible_in_tree() || !is_inside_tree())
 		return String();
 
 	if (!navpoly.is_valid()) {