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
+}