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)

core/io/ip.cpp

@@ -71,7 +71,7 @@ struct _IP_ResolverPrivate {
 	Mutex mutex;
 	Semaphore sem;
 
-	Thread *thread;
+	Thread thread;
 	//Semaphore* semaphore;
 	bool thread_abort;
 
@@ -141,7 +141,7 @@ IP::ResolverID IP::resolve_hostname_queue_item(const String &p_hostname, IP::Typ
 	} else {
 		resolver->queue[id].response = IP_Address();
 		resolver->queue[id].status = IP::RESOLVER_STATUS_WAITING;
-		if (resolver->thread) {
+		if (resolver->thread.is_started()) {
 			resolver->sem.post();
 		} else {
 			resolver->resolve_queues();
@@ -285,26 +285,14 @@ IP::IP() {
 	singleton = this;
 	resolver = memnew(_IP_ResolverPrivate);
 
-#ifndef NO_THREADS
-
 	resolver->thread_abort = false;
-
-	resolver->thread = Thread::create(_IP_ResolverPrivate::_thread_function, resolver);
-#else
-	resolver->thread = nullptr;
-#endif
+	resolver->thread.start(_IP_ResolverPrivate::_thread_function, resolver);
 }
 
 IP::~IP() {
-#ifndef NO_THREADS
-	if (resolver->thread) {
-		resolver->thread_abort = true;
-		resolver->sem.post();
-		Thread::wait_to_finish(resolver->thread);
-		memdelete(resolver->thread);
-	}
-
-#endif
+	resolver->thread_abort = true;
+	resolver->sem.post();
+	resolver->thread.wait_to_finish();
 
 	memdelete(resolver);
 }