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/core_bind.h

@@ -553,7 +553,7 @@ protected:
 	volatile bool active = false;
 	Object *target_instance = nullptr;
 	StringName target_method;
-	Thread *thread = nullptr;
+	Thread thread;
 	static void _bind_methods();
 	static void _start_func(void *ud);
 
@@ -569,9 +569,6 @@ public:
 	String get_id() const;
 	bool is_active() const;
 	Variant wait_to_finish();
-
-	_Thread() {}
-	~_Thread();
 };
 
 VARIANT_ENUM_CAST(_Thread::Priority);