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)

servers/physics_2d/physics_server_2d_wrap_mt.h

@@ -53,7 +53,7 @@ class PhysicsServer2DWrapMT : public PhysicsServer2D {
 	Thread::ID server_thread;
 	Thread::ID main_thread;
 	volatile bool exit;
-	Thread *thread;
+	Thread thread;
 	volatile bool step_thread_up;
 	bool create_thread;