Adding initial playback bar logic.

Harmonia.UI/Converters/NullVisibilityConverter.cs

@@ -0,0 +1,29 @@
+using Avalonia.Data.Converters;
+using System;
+using System.Collections;
+using System.Globalization;
+
+namespace Harmonia.UI.Converters;
+
+public sealed class NullVisibilityConverter : IValueConverter
+{
+    public NullVisibilityConverter()
+    {
+
+    }
+
+    public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+    {
+        if (value is IList list)
+        {
+            return list.Count > 0;
+        }
+
+        return value is not null;
+    }
+
+    public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+    {
+        throw new NotImplementedException();
+    }
+}