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.Mania/Edit/ManiaBeatSnapGrid.cs

@@ -101,7 +101,7 @@ namespace osu.Game.Rulesets.Mania.Edit
                 foreach (var line in grid.Objects.OfType<DrawableGridLine>())
                     availableLines.Push(line);
 
-                grid.Clear(false);
+                grid.Clear();
             }
 
             if (selectionTimeRange == null)