Add basic song select setup for skinnability

osu.Game/Screens/Select/SongSelect.cs

@@ -37,6 +37,7 @@ using osu.Game.Graphics.UserInterface;
 using System.Diagnostics;
 using osu.Game.Screens.Play;
 using osu.Game.Database;
+using osu.Game.Skinning;
 
 namespace osu.Game.Screens.Select
 {
@@ -235,6 +236,10 @@ namespace osu.Game.Screens.Select
                         }
                     }
                 },
+                new SkinnableTargetContainer(SkinnableTarget.SongSelect)
+                {
+                    RelativeSizeAxes = Axes.Both,
+                },
             });
 
             if (ShowFooter)

osu.Game/Skinning/DefaultSkin.cs

@@ -70,6 +70,14 @@ namespace osu.Game.Skinning
                 case SkinnableTargetComponent target:
                     switch (target.Target)
                     {
+                        case SkinnableTarget.SongSelect:
+                            var songSelectComponents = new SkinnableTargetComponentsContainer(container =>
+                            {
+                                // do stuff when we need to.
+                            });
+
+                            return songSelectComponents;
+
                         case SkinnableTarget.MainHUDComponents:
                             var skinnableTargetWrapper = new SkinnableTargetComponentsContainer(container =>
                             {

osu.Game/Skinning/SkinnableTarget.cs

@@ -5,6 +5,7 @@ namespace osu.Game.Skinning
 {
     public enum SkinnableTarget
     {
-        MainHUDComponents
+        MainHUDComponents,
+        SongSelect
     }
 }