Add a filter event to the check if an article is already in the basket
Use notify event instead of filter event
Extract method
Merge pull request #1708 in SW/shopware from feature/master/add-check-basket-event to master

* commit 'db2f1d7412b6927a323b8d56bc769105f8644dea':
  Extract method
  Use notify event instead of filter event
  Add a filter event to the check if an article is already in the basket