[GSoC] Formula Editor Lists

https://jenkins.catrob.at/job/Catroid-Multi-Job-Custom-Branch/2442/
I just talked with Jonny from the UX team. He confirmed that the UX-team's decision was to call this neither String nor Word, but instead ABC in the formula editor. Please change it. The identifier should also not be named formula_editor_word but either formula_editor_ABC or formula_editor_string (please decide what seems easier to understand for future developers; I would say the latter, but I may be wrong).