Implement move [left|right|up|down]

The exact semantics of this command are complicated. I'll describe each
test scenario as s-expressions. Everything assumes L_HORIZ if not
specified, but if you rotate everything 90 degrees the same test cases
hold.

```
(container (view a) (view b focus) (view c))
-> move left
(container (view b focus) (view a) (view c))

(container (view a) (view b focus) (view c))
-> move right
(container (view a) (view c) (view b focus))

(container L_VERT (view a))
(container L_HORIZ
  (view b) (view c focus))
-> move up
(container L_VERT
  (view a) (view c focus))
(container L_HORIZ (view b))

(workspace
  (view a) (view b focus) (view c))
-> move up
(workspace [split direction flipped]
  (view b focus)
  (container (view a) (view c)))

(workspace
  (view a) (view b focus) (view c))
-> move down
(workspace [split direction flipped]
  (container (view a) (view c))
  (view b focus)))

Note: outputs use wlr_output_layout instead of assuming that i+/-1 is
the next output in the move direction.

(root
  (output X11-1
    (workspace 1))
  (output X11-2
    (workspace 1 (view a focus) (view b)))))
-> move left
(root
  (output X11-1
    (workspace 1 (view a focus)))
  (output X11-2
    (workspace 1 (view b)))))

(root
  (output X11-1
    (workspace 1
      (container (view a) (view b)))
  (output X11-2
    (workspace 1 (view c focus)))))
-> move left
(root
  (output X11-1
    (workspace 1
      (container (view a) (view b))
      (view c focus)))
  (output X11-2
    (workspace 1)))
```

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