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)

editor/editor_node.h

@@ -108,7 +108,7 @@ public:
 		String path;
 		List<String> args;
 		String output;
-		Thread *execute_output_thread = nullptr;
+		Thread execute_output_thread;
 		Mutex execute_output_mutex;
 		int exitcode = 0;
 		volatile bool done = false;