Widget areas in templates

I made a little modification to Extensions::renderWidgetHolder method, to allow extensions to place widgets on frontend. With this change we can define areas in templates like this:

```twig
{{ widget( 'frontend', 'top_left' ) }}
```

In the extension:

```PHP
$this->addWidget( 'frontend', 'top_left', 'widgetMethod', 'additionalHTML', 3600 );
```

What do you think of this change? Should I make changes to better suit your conception or it can be merged?
I think in future it should be changed to allow ordering of widgets.