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

.idea/inspectionProfiles/Project_Default.xml

@@ -0,0 +1,14 @@
+<component name="InspectionProjectProfileManager">
+  <profile version="1.0">
+    <option name="myName" value="Project Default" />
+    <inspection_tool class="MavenPackageUpdate" enabled="true" level="WARNING" enabled_by_default="true">
+      <option name="excludeList">
+        <list>
+          <option value="org.openjfx:javafx-fxml" />
+          <option value="org.junit.jupiter:junit-jupiter-api" />
+          <option value="org.junit.jupiter:junit-jupiter-engine" />
+        </list>
+      </option>
+    </inspection_tool>
+  </profile>
+</component>