Fix osu! editor padding being applied to toolboxes

The result of https://github.com/ppy/osu/pull/20739 and https://github.com/ppy/osu/pull/20738 having unintended side effects when applied together.

osu.Game/Rulesets/Edit/HitObjectComposer.cs

@@ -104,7 +104,7 @@ namespace osu.Game.Rulesets.Edit
 
             InternalChildren = new Drawable[]
             {
-                new Container
+                PlayfieldContentContainer = new Container
                 {
                     Name = "Content",
                     RelativeSizeAxes = Axes.Both,
@@ -166,6 +166,15 @@ namespace osu.Game.Rulesets.Edit
             EditorBeatmap.SelectedHitObjects.CollectionChanged += selectionChanged;
         }
 
+        /// <summary>
+        /// Houses all content relevant to the playfield.
+        /// </summary>
+        /// <remarks>
+        /// Generally implementations should not be adding to this directly.
+        /// Use <see cref="LayerBelowRuleset"/> or <see cref="BlueprintContainer"/> instead.
+        /// </remarks>
+        protected Container PlayfieldContentContainer { get; private set; }
+
         protected override void LoadComplete()
         {
             base.LoadComplete();