Formula string extension and refactoring

* UX-changes "String" -> "ABC"
* Dialog UI-tests for ABC(~string) creation.
* Refactoring of change- and set-actions.begin().

Jenkins testrun (pending): 
https://jenkins.catrob.at/view/All-Categories/view/Catroid-multi-job/job/Catroid-Multi-Job-Custom-Branch/2392/

The note brick uses `edit_text`, some of yours `edittext`, if possible, try to be consistent (I don't know which one we use more often, but personally, I'd say `edit_text` since it's `EditText`.