Implemented ObjectProperty for variable in Item, Income and in Expense class.

In Item class the variable date now uses ObjectProperty, and for IncomeCategory and ExpenseCategory in Income and Expense also use ObjectProperty. This is to make the code function better with the frontend.

src/main/java/no/ntnu/idatt1002/demo/data/Economics/Expense.java

 package no.ntnu.idatt1002.demo.data.Economics;
 
+import javafx.beans.property.ObjectProperty;
+import javafx.beans.property.SimpleObjectProperty;
+
 import java.time.LocalDate;
 import java.util.Objects;
 
 public class Expense extends Item{
-    private ExpenseCategory category;
+    private ObjectProperty<ExpenseCategory> category;
 
     /**
      * This constructor uses the super constructor to set the fields for 'amount' and 'recurring' and then sets the
@@ -22,7 +25,7 @@ public class Expense extends Item{
         if(category == null) {
             throw new IllegalArgumentException("The income must belong to a category.");
         }
-        this.category = category;
+        this.category = new SimpleObjectProperty<>(category);
     }
 
     /**
@@ -39,8 +42,7 @@ public class Expense extends Item{
         if(category == null) {
             throw new IllegalArgumentException("The income must belong to a category.");
         }
-        this.category = category;
-
+        this.category = new SimpleObjectProperty<>(category);
     }
 
     /**
@@ -48,7 +50,7 @@ public class Expense extends Item{
      * @return The category the Expense belongs to as a value of the ExpenseCategory enum class.
      */
     public ExpenseCategory getCategory() {
-        return category;
+        return category.get();
     }
 
     /**
@@ -59,7 +61,7 @@ public class Expense extends Item{
         if(expenseCategory == null) {
             throw new IllegalArgumentException("The income must belong to a category.");
         }
-        this.category = expenseCategory;
+        this.category.set(expenseCategory);
     }
 
     /**
@@ -69,7 +71,7 @@ public class Expense extends Item{
      */
     @Override
     public String toString() {
-        return super.toString()+"category="+category.toString()+"\n\n";
+        return super.toString()+"category="+category.get()+"\n\n";
     }
 
     @Override
@@ -77,7 +79,7 @@ public class Expense extends Item{
         if (this == o) return true;
         if (!(o instanceof Expense expense)) return false;
         if (!super.equals(o)) return false;
-        return category == expense.category;
+        return category.get() == expense.category.get();
     }
 
     @Override