Improve code readability and structure

- Fix exception when trying to place an item without one selected
- Add access modifiers to all class members
- Add `final` to all variables if applicable
- Add `TestBehaviour` to the machines to test the event system
- Convert classes to records where appropriate
- Change `Function<T, T>` to `Consumer<T>`
- Change non generic methods in the `EventHandler` class to generic methods
- Remove unused imports
- Other minor improvements

target/classes/com/yes/yes/main-view.fxml

@@ -1,10 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
-<?import javafx.geometry.Insets?>
-<?import javafx.scene.control.Label?>
 <?import javafx.scene.layout.VBox?>
-
-<?import javafx.scene.control.Button?>
 <VBox spacing="20.0" xmlns:fx="http://javafx.com/fxml"
       fx:controller="com.yes.yes.MainController" fx:id="root">
 </VBox>