Better use of async functions for API

- Use local_preferences for storing data

lib/preferences.dart

@@ -0,0 +1,41 @@
+import 'package:shared_preferences/shared_preferences.dart';
+import 'api.dart';
+
+
+class InvenTreeUserPreferences {
+
+  static const String _SERVER = 'server';
+  static const String _USERNAME = 'username';
+  static const String _PASSWORD = 'password';
+
+  // Ensure we only ever create a single instance of the preferences class
+  static final InvenTreeUserPreferences _api = new InvenTreeUserPreferences._internal();
+
+  factory InvenTreeUserPreferences() {
+    return _api;
+  }
+
+  InvenTreeUserPreferences._internal();
+
+  // Load saved login details, and attempt connection
+  void loadLoginDetails() async {
+    SharedPreferences prefs = await SharedPreferences.getInstance();
+
+    var server = prefs.getString(_SERVER);
+    var username = prefs.getString(_USERNAME);
+    var password = prefs.getString(_PASSWORD);
+
+    await InvenTreeAPI().connect(server, username, password);
+  }
+
+  void saveLoginDetails(String server, String username, String password) async {
+    SharedPreferences prefs = await SharedPreferences.getInstance();
+
+    await prefs.setString(_SERVER, server);
+    await prefs.setString(_USERNAME, username);
+    await prefs.setString(_PASSWORD, password);
+
+    // Reconnect the API
+    await InvenTreeAPI().connect(server, username, password);
+  }
+}