Refactor variable list

Branch test run:
http://catrobatgw.ist.tugraz.at:8080/job/Catroid-custom-branch-test/983/testReport/

Rerun of failed tests with master and refactor_variable_list_branch (exept the web tests):
http://catrobatgw.ist.tugraz.at:8080/job/Catroid-single-UI-package-test/115/ (refactor_variable_list_branch)
http://catrobatgw.ist.tugraz.at:8080/job/Catroid-single-UI-package-test/116/testReport/? (master)