Implementing 0-duration tweens

Some light refactor
Adding comments in functions

scene/animation/tween.h

@@ -135,6 +135,7 @@ private:
 	void _tween_process(float p_delta);
 	void _remove_by_uid(int uid);
 	void _push_interpolate_data(InterpolateData &p_data);
+	bool _build_interpolation(InterpolateType p_interpolation_type, Object *p_object, NodePath *p_property, StringName *p_method, Variant p_initial_val, Variant p_final_val, real_t p_duration, TransitionType p_trans_type, EaseType p_ease_type, real_t p_delay);
 
 protected:
 	bool _set(const StringName &p_name, const Variant &p_value);