Make compose mode the default

Hopefully we can keep this at a ScreenSelectionTabControl level, but it may need to be moved to the Editor at some point. I'm leaving that for a future change however, if it's needed.

osu.Game/Screens/Edit/Menus/ScreenSelectionTabControl.cs

@@ -33,6 +33,8 @@ public ScreenSelectionTabControl()
                 Height = 1,
                 Colour = Color4.White.Opacity(0.2f),
             });
+
+            Current.Value = EditorScreenMode.Compose;
         }
 
         [BackgroundDependencyLoader]