Added bindable to adjust playback speed

osu.Game/Configuration/OsuConfigManager.cs

@@ -70,6 +70,7 @@ protected override void InitialiseDefaults()
             Set(OsuSetting.KeyOverlay, false);
 
             Set(OsuSetting.FloatingComments, false);
+            Set(OsuSetting.PlaybackSpeed, 1.0, 0.5f, 2);
 
             // Update
 
@@ -91,6 +92,7 @@ public enum OsuSetting
         DimLevel,
         KeyOverlay,
         FloatingComments,
+        PlaybackSpeed,
         ShowInterface,
         MouseDisableButtons,
         MouseDisableWheel,

osu.Game/Screens/Play/Options/PlaybackOptions.cs

@@ -17,7 +17,7 @@ private void load(OsuConfigManager config)
             Add(new SettingsSlider<double>
             {
                 LabelText = "Playback speed",
-                Bindable = config.GetBindable<double>(OsuSetting.DimLevel),
+                Bindable = config.GetBindable<double>(OsuSetting.PlaybackSpeed),
             });
         }
     }