add opened slns to previous sln list

src/SharpIDE.Godot/Features/IdeSettings/AppState.cs

@@ -4,7 +4,7 @@ public class AppState
 {
     public string? LastOpenSolutionFilePath { get; set; }
     public IdeSettings IdeSettings { get; set; } = new IdeSettings();
-    public List<PreviouslyOpenedSln> PreviouslyOpenedSolutions { get; set; } = [];
+    public HashSet<PreviouslyOpenedSln> PreviouslyOpenedSolutions { get; set; } = [];
 }
 
 public class IdeSettings
@@ -12,7 +12,7 @@ public class IdeSettings
     public bool AutoOpenLastSolution { get; set; }
 }
 
-public class PreviouslyOpenedSln
+public record PreviouslyOpenedSln
 {
     public required string Name { get; set; }
     public required string FilePath { get; set; }

src/SharpIDE.Godot/IdeWindow.cs

@@ -79,6 +79,7 @@ public partial class IdeWindow : Control
                 return;
             }
             ideRoot.SetSlnFilePath(slnPath);
+            Singletons.AppState.PreviouslyOpenedSolutions.Add(new PreviouslyOpenedSln { FilePath = slnPath, Name = Path.GetFileName(slnPath) });
             
             await this.InvokeAsync(() =>
             {