Make resize grow|shrink width|height work for a nested split in the same direction.

Suppose a horizontally oriented workspace has two windows open. Issuing "split h" on
the right window creates a nested horizontal container. If we now resize in "width"
direction, the resize doesn't work because it will only operate on the current parent,
which is a container with only one child, so the resize command exits.

This is unexpected behavior from a user point of view. Hence, with this patch, we
ensure that we don't just go up the tree until we find a parent with the correct
orientation, but also keep going if that parent has only a single child.

fixes #2015