Added WinUI project.

Harmonia.WinUI/Converters/SecondsToStringConverter.cs

@@ -0,0 +1,30 @@
+using Microsoft.UI.Xaml.Data;
+using System;
+
+namespace Harmonia.WinUI.Converters;
+
+public sealed partial class SecondsToStringConverter : IValueConverter
+{
+    public SecondsToStringConverter()
+    {
+
+    }
+
+    public object? Convert(object value, Type targetType, object parameter, string language)
+    {
+        if (value is not double doubleValue)
+            return null;
+
+        TimeSpan timeSpan = TimeSpan.FromSeconds(doubleValue);
+
+        if (timeSpan.Hours >= 1)
+            return timeSpan.ToString(@"%h\:mm\:ss");
+
+        return timeSpan.ToString(@"%m\:ss");
+    }
+
+    public object ConvertBack(object value, Type targetType, object parameter, string language)
+    {
+        return TimeSpan.Parse((string)value);
+    }
+}