Add settings page (#46)

Fixes #37, fixes #47.

Reviewed-on: Nexus/nexus#46

lib/models/setting.dart

@@ -0,0 +1,16 @@
+import "package:flutter/material.dart";
+
+class Setting {
+  final String title;
+  final String description;
+  final IconData icon;
+  final Widget Function(String title, String description, IconData icon)
+  builder;
+
+  Setting({
+    required this.title,
+    required this.description,
+    required this.builder,
+    required this.icon,
+  });
+}