Fix restarting with 32bit depth windows (v5)

What I do in this patch is:

1. Split the con_new() function, so I can create a Con without actually
create a window.
2. Store the depth of Cons in the layout file when i3 is restarting.

Fix typos and mis-staged files in previous patch.

include/data.h

@@ -584,6 +584,9 @@ struct Con {
     /* The ID of this container before restarting. Necessary to correctly
      * interpret back-references in the JSON (such as the focus stack). */
     int old_id;
+
+    /* Depth of the container window */
+    uint16_t depth;
 };
 
 #endif