Refactor drawable hierarchy to center properly

* Use FillFlowContainer.Spacing instead of manually applying margins.
* Use Update() for calculating button padding to preserve it after
  mod button expansion and adjust FooterButtonRandom to use this method
  while avoiding flickering.
* Expose mod display margin to clear it in the footer button.

osu.Game/Screens/Select/FooterButtonRandom.cs

@@ -24,8 +24,11 @@ namespace osu.Game.Screens.Select
                 Anchor = Anchor.Centre,
                 Origin = Anchor.Centre,
                 Text = @"rewind",
-                Alpha = 0
+                Alpha = 0,
             });
+
+            // force both text sprites to always be present to avoid width flickering while they're being swapped out
+            SpriteText.AlwaysPresent = secondaryText.AlwaysPresent = true;
         }
 
         [BackgroundDependencyLoader]