add unfolded=yes/no key in gui2 treeview node wml

This allows wml to specify whether a node should be unfolded by default.

src/gui/auxiliary/window_builder/tree_view.cpp

@@ -79,7 +79,9 @@ twidget* tbuilder_tree_view::build() const
 }
 
 tbuilder_tree_view::tnode::tnode(const config& cfg)
-	: id(cfg["id"]), builder(NULL)
+	: id(cfg["id"])
+	, unfolded(cfg["unfolded"].to_bool(false))
+	, builder(NULL)
 {
 	VALIDATE(!id.empty(), missing_mandatory_wml_key("node", "id"));
 

src/gui/auxiliary/window_builder/tree_view.hpp

@@ -43,6 +43,7 @@ struct tbuilder_tree_view : public tbuilder_control
 		explicit tnode(const config& cfg);
 
 		std::string id;
+		bool unfolded;
 		tbuilder_grid_ptr builder;
 	};
 

src/gui/widgets/tree_view_node.cpp

@@ -74,6 +74,11 @@ ttree_view_node::ttree_view_node(
 							&ttree_view_node::signal_handler_left_button_click,
 							this,
 							_2), event::tdispatcher::back_post_child);
+
+					if(node_definition.unfolded) {
+						toggle_->set_value(1);
+						unfolded_ = true;
+					}
 				}
 
 				if(parent_node_ && parent_node_->toggle_) {