Store the active View instance

This is other approach of https://github.com/fuel/core/pull/1380

The rendering process is not in scope of view instance.
So can not access as ```$this```.
And can not current view instance delivery to rendering scope as $this.
(If a View instance is passed as $this, it will not operate normally by behavior of PHP. )

I think, the approach passing as a variable is contamination of variable in scope of rendering.

This approach makes it possible to access an instance of the current view.

```php
<body>
   <div id="container">
       <?php echo View::forge('next/view', View::active()->get()) ?>
   </div>
</body>
```

If you agree to this approach, I'll prepare tests and docs.