Create interfaces for DownloadTrackingComposite to consume

osu.Game/Database/IModelManager.cs

@@ -0,0 +1,20 @@
+
+using System;
+
+namespace osu.Game.Database
+{
+    public interface IModelManager<TModel>
+    {
+        /// <summary>
+        /// Fired when a new <see cref="TModel"/> becomes available in the database.
+        /// This is not guaranteed to run on the update thread.
+        /// </summary>
+        event Action<TModel, bool> ItemAdded;
+
+        /// <summary>
+        /// Fired when a <see cref="TModel"/> is removed from the database.
+        /// This is not guaranteed to run on the update thread.
+        /// </summary>
+        event Action<TModel> ItemRemoved;
+    }
+}