Star favourite sceneries in the list

A right click stars or unstars the scenery under it; the starred ones are
gathered in a group on top and marked where they belong. They are kept in
favorites.txt beside the settings, not in eu07.ini.

StarterNG/Application/FavoriteSceneries.cs

@@ -0,0 +1,72 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using StarterNG.Application.Abstractions;
+using StarterNG.Domain.Sceneries;
+
+namespace StarterNG.Application;
+
+/// <summary>
+/// The sceneries starred in the list, by file name, one per line in a file of the
+/// starter's own - eu07.ini is the simulator's as much as ours.
+/// </summary>
+public sealed class FavoriteSceneries
+{
+    private readonly IFileSystem _files;
+    private readonly IDiagnosticsLog _log;
+    private readonly string _path;
+    private readonly HashSet<string> _names = new(StringComparer.OrdinalIgnoreCase);
+
+    public FavoriteSceneries(IFileSystem files, IDiagnosticsLog log, string path)
+    {
+        _files = files;
+        _log = log;
+        _path = path;
+        Load();
+    }
+
+    public bool Contains(Scenery scenery) => _names.Contains(Key(scenery));
+
+    public void Toggle(Scenery scenery)
+    {
+        string key = Key(scenery);
+        if (!_names.Remove(key))
+            _names.Add(key);
+        Save();
+    }
+
+    private static string Key(Scenery scenery) => Path.GetFileName(scenery.Path);
+
+    private void Load()
+    {
+        try
+        {
+            if (!_files.FileExists(_path))
+                return;
+
+            foreach (string line in _files.ReadAllText(_path).Split('\n'))
+                if (line.Trim() is { Length: > 0 } name)
+                    _names.Add(name);
+        }
+        catch (Exception ex)
+        {
+            _log.Log($"reading {_path}", ex);
+        }
+    }
+
+    private void Save()
+    {
+        try
+        {
+            if (Path.GetDirectoryName(_path) is { Length: > 0 } directory)
+                _files.CreateDirectory(directory);
+
+            _files.WriteAllText(_path, string.Join('\n', _names.Order(StringComparer.OrdinalIgnoreCase)));
+        }
+        catch (Exception ex)
+        {
+            _log.Log($"writing {_path}", ex);
+        }
+    }
+}