Flatten tree

[polycatenane]

closes #944

[mahkoh]

Please make this a tristate Never, Always, OnRemove where OnRemove only takes effect when the second-to-last child of a container is removed.

This requires adding a non_exhaustive enum to jay-config and a corresponding enum in Jay proper.

I think this should also be under the [theme] table in the toml config.

You also have to add a corresponding setting in the look-and-feel pane in the control center.

[polycatenane]

Is the difference between Always and OnRemove that OnRemove lets you manually nest a single child deep in a tree? Always should be closest to how i3 behaves.

[mahkoh]

Is the difference between Always and OnRemove that OnRemove lets you manually nest a single child deep in a tree?

Yes.

[mahkoh]

Please follow the style of the rest of the codebase. Also squash the commits as appropriate.