Refactor to add header to the Tracked Item pages.

lib/models/item_type.dart

@@ -2,21 +2,26 @@ enum ItemType {
   expense(
     title: "Expense",
     plural: "Expenses",
+    description: "Items which cost revenue, or decrease asset value.",
   ),
   income(
     title: "Income",
-    plural: "Incomes",
+    plural: "Income",
+    description: "Items which bring in revenue, or increase asset value.",
   ),
   asset(
     title: "Asset",
     plural: "Assets",
+    description: "Value which has been earned and can be spent.",
   );
 
   const ItemType({
     required this.title,
     required this.plural,
+    required this.description,
   });
 
   final String title;
   final String plural;
+  final String description;
 }