Fix attribute options store label and add support translation

[bleleve]

Hello,

Currently it is not possible to set options labels per store for dropdown and multi-select attributes. Each store label is added as a new option, which is not the desired behaviour. This PR fix this.

Be careful, the number of attribute options must be the same for all stores (including the admin store), and they must also be in the same order.

Example for a multi-select attribute :

array(
    array(
        'sku' => 'TESTSKU',
        'test_multi_attr' => 'Option1-AdminStoreLabel', /* Option 1 - Admin Store Label */
    ),
    array(
        'sku' => null,
        'test_multi_attr' => 'Option2-AdminStoreLabel', /* Option 2 - Admin Store Label */
    ),
    array(
        'sku' => null,
        '_store' => 'default',
        'test_multi_attr' => 'Option1-DefaultStoreLabel', /* Option 1 - Default Store Label */
    ),
    array(
        'sku' => null,
        '_store' => 'default',
        'test_multi_attr' => 'Option2-DefaultStoreLabel', /* Option 2 - Default Store Label */
    )
)