use more fitting floating action button, cleanup

frontend/lib/modules/current_trip_save_button.dart

@@ -0,0 +1,33 @@
+
+import 'package:anyway/structs/trip.dart';
+import 'package:auto_size_text/auto_size_text.dart';
+import 'package:flutter/material.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+Widget saveButton(Trip trip) => ElevatedButton(
+  onPressed: () async {
+    SharedPreferences prefs = await SharedPreferences.getInstance();
+    trip.toPrefs(prefs);
+  },
+  child: SizedBox(
+    width: 100,
+    child: Row(
+      mainAxisAlignment: MainAxisAlignment.center,
+      children: [
+        Icon(
+          Icons.save,
+        ),
+        Expanded(
+          child: Padding(
+            padding: EdgeInsets.only(left: 10, top: 5, bottom: 5, right: 5),
+            child: AutoSizeText(
+              'Save trip',
+              maxLines: 2,
+            ),
+          ),
+        ),
+      ],
+    ),
+  )
+);
+