Initial commit

System that behaves like i3 on one screen. Supports nesting, moving
focus, and moving windows like i3. Doesn't support stacking, tabbing,
resizing, mousing, or selecting parents yet. Also doesn't support any
"auxiliary" i3 features.
Remember last selection at every node

This can help us remember where a window was if we move it out of a
container and then directly back in, for instance. i3 seems to do this,
and this mimics its behavior perfectly.
Add selectParent, selectChild, and closeSelected ops
Allow moving window perpendicular to top-level container

This creates a new top-level container. A very convenient feature taken
from i3.

Now, the 'root' of the layout is distinct from the top-level node. I
might make them distinct classes in the future.

The behavior of this WM now mimics the behavior of i3 almost perfectly.
There is a very slight difference in the way this feature behaves in
some situations, but it's pretty esoteric.