Refactor logging

- Add warning log level
- Change debug_log_level setting to enumeration string
- Map Irrlicht log events to MT log events
- Encapsulate log_* functions and global variables into a class, Logger
- Unify dstream with standard logging mechanism
- Unify core.debug() with standard core.log() script API

src/threading/thread.cpp

@@ -222,11 +222,11 @@ void *Thread::theThread(void *param)
 	th->running = true;
 
 	th->setName();
-	log_register_thread(th->name);
+	g_logger.registerThread(th->name);
 
 	th->retval = th->run();
 
-	log_deregister_thread();
+	g_logger.deregisterThread();
 
 	th->running = false;
 #if __cplusplus < 201103L