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/

Again, keep formula_editor_keyboard_string