single slicer instance

check for other instances during startup
send message with command line arguments if found and terminate
listen for those messages and load objects from paths in messages from them

src/slic3r/GUI/AppConfig.cpp

@@ -69,6 +69,9 @@ void AppConfig::set_defaults()
         set("use_retina_opengl", "1");
 #endif
 
+	if (get("single_instance").empty())
+		set("single_instance", "0");
+
     if (get("remember_output_path").empty())
         set("remember_output_path", "1");