Seek at 4x normal speed when holding shift

This matches osu-stable 1:1. Not sure if it feels better or not but
let's stick with what people are used to for the time being.

osu.Game/Screens/Edit/Editor.cs

@@ -589,7 +589,7 @@ namespace osu.Game.Screens.Edit
 
         private void seek(UIEvent e, int direction)
         {
-            double amount = e.ShiftPressed ? 2 : 1;
+            double amount = e.ShiftPressed ? 4 : 1;
 
             if (direction < 1)
                 clock.SeekBackward(!clock.IsRunning, amount);