Created new class for adding, editing and deleting expenses

src/main/java/no/ntnu/idatt1002/demo/view/ExpenseDialog.java

+package no.ntnu.idatt1002.demo.view;
+
+import javafx.scene.control.Dialog;
+import no.ntnu.idatt1002.demo.data.Expense;
+
+public class ExpenseDialog extends Dialog<Expense> {
+
+}