Associations Interfaces

This pull request introduces the concept of association interfaces to the Symphony core. It doesn't add interfaces itself but it offers extensions a standardised way to do so. In the extension driver, developers can now define the following:

```php
    public static function registerProviders()
    {
        self::$provides = array(
            'association-interface' => array(
                'handle' => 'Name'
            )
            'association-editor' => array(
                'handle' => 'Name'
            )
        );

        return true;
    }
```

Association interfaces are meant to enhance the way associations are displayed to the user.
Association editors are meant to provide a way to manipulate the related data itself.

Interface and editor settings are stored globally in `tbl_sections_associations` and associative fields can make use of a widget function to provide a selection in the section editor:

```php
$this->appendAssociationInterfaceSelect($wrapper);
```

The association information stored in the database will be automatically added to all associative field instances, e. g.:

```html
<div id="field-123" class="field field-select" data-parent-section-id="1" data-parent-section-field-id="104" data-child-section-id="5" data-child-section-field-id="108" data-interface="handle-of-ui" data-editor="handle-of-editor">…</div>
```

Additionally all association data is provided in the Symphony JavaScript object via `Symphony.Context.get('env')`.
Moved it to the `Field` class because it's more consistent.