[Need More Tests] Use RuntimeInitializeOnLoadMethod to call Init()

[UlyssesWu]

This PR implements the method mentioned by @MorganMoon at here.

It no longer uses Awake() for init, the new way is to find and init all bindings before scene loaded:

        static void SceneManager_SceneLoaded(Scene scene, LoadSceneMode mode)
        {
            foreach (var parentT in scene.GetRootGameObjects())
            {
                //Call Init() on all bindings
                foreach (var binding in parentT.GetComponentsInChildren<AbstractMemberBinding>(true)) //including inactive GameObjects
                {
                    binding.Init();
                }
            }
        }

By this way, we can implement ToggleSelfBinding which can toggle active for the gameobject it attached on, even if the gameobject is inactive at startup.

I have tested on Unity-Weld-Examples and all demos should still work. However I don't know if this will bring down the performance by traversing all gameojbects in scene. Maybe a performance test is needed .

[UlyssesWu]

Demo

ViewModel:

    [Binding]
    public bool IsActive
    {
        get { return _isActive; }
        set
        {
            if (value == _isActive) return;
            _isActive = value;
            OnPropertyChanged("IsActive");
        }
    }

TwoWayPropertyBinding (IsActive) on Toggle, ToggleSelfBinding (IsActive) on Text. Text is inactive.

You can still activate Text by switching the Toggle, because the ToggleSelfBinding is initialized by SceneManager_SceneLoaded.

[RoryDungan]

This looks pretty good! Will need some time to test it but so far I think it looks like a good change. I'm away from the office right now but will be back next week so we'll see if one of the other guys has time to test it, otherwise I'll take a more in-depth look at it then.

[RoryDungan]

Should just delete this. We try to avoid leaving blocks of commented-out code because it can be confusing to someone else reading the code.