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) {