better visual coherence

frontend/lib/structs/preferences.dart

@@ -1,3 +1,4 @@
+import 'package:anyway/structs/landmark.dart';
 import 'package:flutter/material.dart';
 
 
@@ -28,27 +29,27 @@ class UserPreferences {
     slug: "sightseeing",
     description: "How much do you like sightseeing?",
     value: 0,
-    icon: Icon(Icons.church),
+    icon: typeSightseeing.icon,
   );
   SinglePreference shopping = SinglePreference(
     name: "Shopping",
     slug: "shopping",
     description: "How much do you like shopping?",
     value: 0,
-    icon: Icon(Icons.shopping_bag),
+    icon: typeShopping.icon,
   );
   SinglePreference nature = SinglePreference(
     name: "Nature",
     slug: "nature",
     description: "How much do you like nature?",
     value: 0,
-    icon: Icon(Icons.landscape),
+    icon: typeNature.icon,
   );
 
   SinglePreference maxTime = SinglePreference(
     name: "Trip duration",
     slug: "duration",
-    description: "How long do you want your trip to be?",
+    description: "How long should your trip be?",
     value: 30,
     minVal: 30,
     maxVal: 720,