Switched to boost::signals2

Index: src/editor/message_box.hpp
--- src/editor/message_box.hpp.orig
+++ src/editor/message_box.hpp
@@ -45,7 +45,7 @@ class MessageBox : public GUI::GroupComponent (public)
   void on_cancel_button();
 
 public:
-  boost::signal<void()> on_ok;
+  boost::signals2::signal<void()> on_ok;
 
 private:
   MessageBox(const MessageBox&);
