quadtree: Try to keep the tree in better balance.

quadtree: Try to keep the tree in better balance.

When a tree node is removed, we now attempt to add the detached
subtree in more optimized order to the new tree. This is done so that
we walk down the detached tree and add a new node to the main tree
always when the tree is one quarter'th smaller than the previous node
that was added.

Signed-off-by: Timo Kokkonen <kaapeli@itanic.dy.fi>