Sloppy naming between window/container?

[thblt]

As the title says. Some functions talk about containers (eg sway-focus-containers), while others speak of windows (sway-list-windows). In the latter example, window probably means final container, or a leaf in the sway tree — but it's worth making sure we're using names consistently in the future.

Proposed definition:

— Window: terminal container, tree leaf.
— Container: both leaves and nodes
— A name for pure nodes? That is, containers that aren't windows (sway containers)