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)))
```

1 files changed, 4 insertions, 0 deletions

diff --git a/sway/commands/move.c b/sway/commands/move.c
index c954ab94..15a5ebc4 100644
--- a/sway/commands/move.c
+++ b/sway/commands/move.c
@@ -82,6 +82,8 @@ static struct cmd_results *cmd_move_container(struct sway_container *current,
                                 config->handler_context.seat, ws);
                 container_move_to(current, focus);
                 seat_set_focus(config->handler_context.seat, old_parent);
+                container_reap_empty(old_parent);
+                container_reap_empty(focus->parent);
                 return cmd_results_new(CMD_SUCCESS, NULL, NULL);
         } else if (strcasecmp(argv[1], "to") == 0
                         && strcasecmp(argv[2], "output") == 0) {
@@ -109,6 +111,8 @@ static struct cmd_results *cmd_move_container(struct sway_container *current,
                 struct sway_container *old_parent = current->parent;
                 container_move_to(current, focus);
                 seat_set_focus(config->handler_context.seat, old_parent);
+                container_reap_empty(old_parent);
+                container_reap_empty(focus->parent);
                 return cmd_results_new(CMD_SUCCESS, NULL, NULL);
         }
         return cmd_results_new(CMD_INVALID, "move", expected_syntax);