Mostly refactoring

src/main/java/ch/hepia/events/ChatMessage.java

src/main/java/ch/hepia/events/Event.java

src/main/java/ch/hepia/events/JoinedJourney.java

src/main/java/ch/hepia/events/LeftJourney.java

src/main/java/ch/hepia/ui/MainWindowController.java

src/main/resources/fxml/MainWindow.fxml