added difficulty class and dropdown

lib/models/difficulty.dart

@@ -0,0 +1,8 @@
+class Difficulty {
+  static List<String> get difficulties {
+    return List<String>.generate(_Difficulty.values.length,
+        (index) => _Difficulty.values.elementAt(index).name);
+  }
+}
+
+enum _Difficulty { veryEasy, easy, intermediate, hard, veryHard }