Modernize Thread

- Based on C++11's `thread` and `thread_local`
- No more need to allocate-deallocate or check for null
- No pointer anymore, just a member variable
- Platform-specific implementations no longer needed (except for the few cases of non-portable functions)
- Simpler for `NO_THREADS`
- Thread ids are now the same across platforms (main is 1; others follow)

main/main.cpp

@@ -359,8 +359,6 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 
 	MAIN_PRINT("Main: Initialize Globals");
 
-	Thread::_main_thread_id = Thread::get_caller_id();
-
 	globals = memnew(ProjectSettings);
 	input_map = memnew(InputMap);
 
@@ -1275,9 +1273,11 @@ Error Main::setup2(Thread::ID p_main_tid_override) {
 	// Print engine name and version
 	print_line(String(VERSION_NAME) + " v" + get_full_version_string() + " - " + String(VERSION_WEBSITE));
 
+#if !defined(NO_THREADS)
 	if (p_main_tid_override) {
-		Thread::_main_thread_id = p_main_tid_override;
+		Thread::main_thread_id = p_main_tid_override;
 	}
+#endif
 
 	Error err = OS::get_singleton()->initialize(video_mode, video_driver_idx, audio_driver_idx);
 	if (err != OK) {