add(ui): add credit scene and some other small changes i actually forgot to write down #56

...m_stuttgart/battlearena/Controller/CreditsController.java

...hdm_stuttgart/battlearena/Controller/IntroController.java

..._stuttgart/battlearena/Controller/MainMenuController.java

...m_stuttgart/battlearena/Controller/OptionsController.java

src/main/java/de/hdm_stuttgart/battlearena/Main/Main.java

src/main/resources/fxml/Credits.fxml

src/main/resources/textures/videos/credits.mp4