Do detached children leak memory due to parent/child cycle

[yjbanov]

Reported by @mraleph. Reading the code, there should be a memory leak when detaching a sub-tree of RenderNodes. Despite Detach() nulling out _parent for the top-most detached child, the cycles inside the sub-tree should still prevent the destructors from being called. A quick speculative fix is to override Detach() in RenderMultiChildParent and call Detach() recursively on all live children.

[yjbanov]

Or simply make child._parent a weak_ptr.