Formula string extension and refactoring

* UX-changes "String" -> "Word"
* Dialog UI-tests for word (~string) creation.
* Refactoring of change- and set-actions.begin().

Jenkins testrun (green): 
https://jenkins.catrob.at/view/All-Categories/view/Catroid-multi-job/job/Catroid-Multi-Job-Custom-Branch/2314/
Of course! ;)