Fixed a ConcurrentModificationException

Concerning #245 

There was a problem, while looping through a arraylist with a for each loop and deleting elements from the list. Now the looping and removing is done with a iterator.

I added a regression test for this failure: MultipleBroadcastsTest

Jenkins Testrun:
http://catrobatgw.ist.tugraz.at:8080/job/Catroid-custom-branch-test/747/
SrcTests (for licence and assertions):
http://catrobatgw.ist.tugraz.at:8080/job/Catroid-custom-branch-test-source-and-jUnit-tests-only/49/
5 iterations of the MultipleBroadcastsTest:
http://catrobatgw.ist.tugraz.at:8080/job/Catroid-single-UI-test/502/