Fixes mulitple event triggering on action forwarding.

Fixes this issue:

https://github.com/ShopwareAG/shopware-4/issues/45
http://jira.shopware.de/Widgets/Jira/?ticket=SW-5158


The action dispatcher does not check if the `_dispatched` property of the given request has changed during the action processing.

The check is included in this pull request.