Fix broadcast wait blocking behavior

Broadcasts now behave exactly the same as in Scratch.
A BroadcastWait 'message' brick waiting for a When 'message' Script to complete is unblocked when 'message' is sent again from another Script (the When 'message' script is restarted).
See [this cucumber feature](https://github.com/Catrobat/Catroid/blob/CucumberPlayground/catroidCucumberTest/assets/features/bricks/BroadcastWaitBlockingBehavior.feature) for more examples.

Fixes #709.
Jenkins testrun: [920](https://jenkins.catrob.at/job/Catroid-Multi-Job-Custom-Branch/920/)