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,
+  });
+}

lib/models/settings.dart

@@ -0,0 +1,16 @@
+import "package:flutter/material.dart";
+import "package:freezed_annotation/freezed_annotation.dart";
+part "settings.freezed.dart";
+part "settings.g.dart";
+
+@freezed
+abstract class Settings with _$Settings {
+  const factory Settings({
+    @Default(ThemeMode.system) ThemeMode theme,
+    @Default(true) bool useDynamicTheming,
+    @Default(false) bool linuxMobileMode,
+  }) = _Settings;
+
+  factory Settings.fromJson(Map<String, Object?> json) =>
+      _$SettingsFromJson(json);
+}

lib/models/settings_category.dart

@@ -0,0 +1,14 @@
+import "package:fast_immutable_collections/fast_immutable_collections.dart";
+import "package:flutter/material.dart";
+import "package:freezed_annotation/freezed_annotation.dart";
+import "package:nexus/models/setting.dart";
+part "settings_category.freezed.dart";
+
+@freezed
+abstract class SettingsCategory with _$SettingsCategory {
+  const factory SettingsCategory({
+    required String title,
+    required IconData icon,
+    required IList<Setting> settings,
+  }) = _SettingsCategory;
+}