Always use LifetimeEntry to manage hit objects in HitObjectContainer

Previously, non-pooled DHOs were immediately added as children of the HOC when Add is called. Also, non-pooled DHOs were always attached to the HOC as children.
New behavior is that non-pooled DHOs are only added after CheckChildLifetime, and only attached to the HOC while the DHOs are alive.

- LifetimeManagementContainer inheritance of HOC is removed, as it is now all DHOs are "unmanaged" (previously `AddInternal(false)`).
- The signature of `Clear` is changed, and it is now always not disposing the children immediately.

osu.Game/Rulesets/Objects/Pooling/PoolableDrawableWithLifetime.cs

@@ -18,7 +18,7 @@ namespace osu.Game.Rulesets.Objects.Pooling
         /// <summary>
         /// The entry holding essential state of this <see cref="PoolableDrawableWithLifetime{TEntry}"/>.
         /// </summary>
-        protected TEntry? Entry { get; private set; }
+        public TEntry? Entry { get; private set; }
 
         /// <summary>
         /// Whether <see cref="Entry"/> is applied to this <see cref="PoolableDrawableWithLifetime{TEntry}"/>.