fix bug that treebox.clear only rm the 1st node

include/nana/gui/widgets/categorize.hpp

@@ -1,7 +1,7 @@
 /**
  *	A Categorize Implementation
  *	Nana C++ Library(http://www.nanapro.org)
- *	Copyright(C) 2003-2016 Jinhao(cnjinhao@hotmail.com)
+ *	Copyright(C) 2003-2017 Jinhao(cnjinhao@hotmail.com)
  *
  *	Distributed under the Boost Software License, Version 1.0. 
  *	(See accompanying file LICENSE_1_0.txt or copy at 

include/nana/gui/widgets/detail/tree_cont.hpp

@@ -1,6 +1,6 @@
 /*
  *	A Tree Container class implementation
- *	Copyright(C) 2003-2016 Jinhao(cnjinhao@hotmail.com)
+ *	Copyright(C) 2003-2017 Jinhao(cnjinhao@hotmail.com)
  *
  *	Distributed under the Boost Software License, Version 1.0.
  *	(See accompanying file LICENSE_1_0.txt or copy at
@@ -76,12 +76,17 @@ namespace detail
 
 			~tree_cont()
 			{
-				clear();
+				clear(&root_);
 			}
 
-			void clear()
+			void clear(node_type* node)
 			{
-				remove(root_.child);
+				while (node->child)
+				{
+					//If there is a sibling of child, the root_.child
+					//will be assigned with the sibling.
+					remove(node->child);
+				}
 			}
 
 			bool verify(const node_type* node) const

include/nana/gui/widgets/treebox.hpp

@@ -201,6 +201,9 @@ namespace nana
 				/// Set the check state, and it returns itself.
 				item_proxy& check(bool);
 
+				/// Clears the child nodes
+				item_proxy& clear();
+
 				/// Return true when the node is expanded  \todo change to expanded ??
 				bool expanded() const;