Fix for the MessageContainer

Concerning #219 

Added:
* implemented a backup method for the message container so if the loaded project fails to load, the broadcasts from the old project are still available
* inserted the backup method in the loadProject method of the ProjectManager class
* junit tests in the MessageContainerTest class

Jenkins Testrun:
http://catrobatgw.ist.tugraz.at:8080/job/Catroid-custom-branch-test/701/
StageDialogTest:
http://catrobatgw.ist.tugraz.at:8080/job/Catroid-single-UI-test/464/