Add ItemType for determining which DB methods should be called based on the screen data. Begin implementing it.

lib/models/tracked_item.dart

@@ -1,8 +1,11 @@
 // Local
+import 'package:flutter_expense_tracker/models/item_type.dart';
+
 import '/models/frequency.dart';
 
 abstract class TrackedItem {
   int? id;
+  ItemType? type;
   String name;
   double amount;
   Frequency? frequency;
@@ -10,6 +13,7 @@ abstract class TrackedItem {
 
   TrackedItem({
     this.id,
+    this.type,
     required this.name,
     required this.amount,
     this.frequency,