This commit does two things: adds a weak reference to the parent
container of each SplitNode.Container and SplitNode.Leaf and moves the
"direction" of a split out of the SplitNode enum and into the
SplitNode.Container struct as a field.
Both changes are required for supporting split resizing. A reference to
the parent in each leaf and split container is needed in order to
traverse upwards through the split tree. If the focused split is not
part of a container that is split along the direction that was requested
to be resized, then we instead try and resize the parent. If the parent
is split along the requested direction, then it is resized
appropriately; otherwise, repeat until the root of the tree is reached.
The direction is needed inside the SplitNode.Container object itself so
that the container knows whether or not it is able to resize itself in
the direction requested by the user. Once the split direction was moved
inside of SplitNode.Container, it became redundant to also have it as
part of the SplitNode enum, so this simplifies things.
