Fix for Issue#307

Concerning #307 

The screenshot is now created at the beginning and it is saved when the user exits the stage.

I haven't done the saving in an asnyc task, because working with Libgdx methods in background threads is very evil.

Jenkins Tesrun (these tests fail also randomly on the current master):
http://catrobatgw.ist.tugraz.at:8080/job/Catroid-custom-branch-test/1019/