Now, `OnAdd` is always invoked after the DHO is added to the container.
This change is required by `ScrollingHitObjectContainer` to compute origin adjusted lifetime when origin position is relative to the parent drawable size.
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.