More work to make the Expense page arbitrary for re-use.

lib/db.dart

@@ -3,6 +3,7 @@
 // SQLite
 import 'dart:io';
 import 'dart:async';
+import 'package:flutter_expense_tracker/models/recurring_tracked_type.dart';
 import 'package:path/path.dart';
 import 'package:path_provider/path_provider.dart';
 import 'package:sqflite/sqflite.dart';
@@ -76,7 +77,7 @@ class DatabaseHelper {
     return expenseList;
   }
 
-  Future<int> addExpense(Expense expense) async {
+  Future<int> addExpense(RecurringTrackedType expense) async {
     Database db = await instance.db;
     return await db.insert(
       "expense",
@@ -93,7 +94,7 @@ class DatabaseHelper {
     );
   }
 
-  Future<int> updateExpense(Expense expense) async {
+  Future<int> updateExpense(RecurringTrackedType expense) async {
     Database db = await instance.db;
     return await db.update(
       "expense",