Add Track as a property to IWorkingBeatmap

This is implemented by `WorkingBeatmap` already, and is much better to use than loading the track every time we need it.

osu.Game/Beatmaps/IWorkingBeatmap.cs

@@ -42,6 +42,11 @@ namespace osu.Game.Beatmaps
         /// </summary>
         ISkin Skin { get; }
 
+        /// <summary>
+        /// Get the loaded audio track instance.
+        /// </summary>
+        Track Track { get; }
+
         /// <summary>
         /// Constructs a playable <see cref="IBeatmap"/> from <see cref="Beatmap"/> using the applicable converters for a specific <see cref="RulesetInfo"/>.
         /// <para>