Added playlist logic.

Harmonia.Core/Events/PlaylistUpdateAction.cs

@@ -0,0 +1,21 @@
+namespace Harmonia.Core.Events;
+
+public enum PlaylistUpdateAction
+{
+    /// <summary>
+    /// An item was added to the collection.
+    /// </summary>
+    Add,
+    /// <summary>
+    /// An item was removed from the collection.
+    /// </summary>
+    Remove,
+    /// <summary>
+    /// An item was moved within the collection.
+    /// </summary>
+    Move,
+    /// <summary>
+    /// The contents of the collection changed dramatically.
+    /// </summary>
+    Reset
+}

Harmonia.Core/Events/PlaylistUpdatedEventArgs.cs

@@ -0,0 +1,12 @@
+using Harmonia.Core.Models;
+
+namespace Harmonia.Core.Events;
+
+public class PlaylistUpdatedEventArgs : EventArgs
+{
+    public required PlaylistUpdateAction Action { get; init; }
+    public required int Index { get; init; }
+    public int? NewIndex { get; init; } = -1;
+    public required int Count { get; init; }
+    public required PlaylistSong[] Songs { get; init; } = [];
+}