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, 25 insertions, 21 deletions

diff --git a/sway/tree/container.c b/sway/tree/container.c
index ab8363bc..ea1c93bb 100644
--- a/sway/tree/container.c
+++ b/sway/tree/container.c
@@ -208,7 +208,7 @@ static void container_root_finish(struct sway_container *con) {
 	wlr_log(L_ERROR, "TODO: destroy the root container");
 }
 
-static bool container_reap_empty(struct sway_container *con) {
+bool container_reap_empty(struct sway_container *con) {
 	switch (con->type) {
 	case C_ROOT:
 	case C_OUTPUT:
@@ -225,14 +225,6 @@ static bool container_reap_empty(struct sway_container *con) {
 		if (con->children->length == 0) {
 			_container_destroy(con);
 			return true;
-		} else if (con->children->length == 1) {
-			struct sway_container *child = con->children->items[0];
-			if (child->type == C_CONTAINER) {
-				container_remove_child(child);
-				container_replace_child(con, child);
-				_container_destroy(con);
-				return true;
-			}
 		}
 	case C_VIEW:
 		break;
@@ -245,6 +237,29 @@ static bool container_reap_empty(struct sway_container *con) {
 	return false;
 }
 
+struct sway_container *container_reap_empty_recursive(
+		struct sway_container *con) {
+	while (con) {
+		struct sway_container *next = con->parent;
+		if (!container_reap_empty(con)) {
+			break;
+		}
+		con = next;
+	}
+	return con;
+}
+
+struct sway_container *container_flatten(struct sway_container *container) {
+	while (container->type == C_CONTAINER && container->children->length == 1) {
+		struct sway_container *child = container->children->items[0];
+		struct sway_container *parent = container->parent;
+		container_replace_child(container, child);
+		container_destroy(container);
+		container = parent;
+	}
+	return container;
+}
+
 struct sway_container *container_destroy(struct sway_container *con) {
 	if (con == NULL) {
 		return NULL;
@@ -283,18 +298,7 @@ struct sway_container *container_destroy(struct sway_container *con) {
 			break;
 	}
 
-	struct sway_container *tmp = parent;
-	while (parent) {
-		tmp = parent->parent;
-
-		if (!container_reap_empty(parent)) {
-			break;
-		}
-
-		parent = tmp;
-	}
-
-	return tmp;
+	return container_reap_empty_recursive(parent);
 }
 
 static void container_close_func(struct sway_container *container, void *data) {