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/

I would keep formula_editor_new_string_name. ABC is only used at the bottom on the button in the formula editor. The internal name (for developers) should be kept as "string". Same at all similar places. For the user, "Text" and "text" should be used, as commented below.