Bugfix: Close containers which are empty due to a move (Thanks fernando)

include/con.h

@@ -75,6 +75,12 @@ Con *con_by_frame_id(xcb_window_t frame);
  */
 Con *con_for_window(i3Window *window, Match **store_match);
 
+/**
+ * Returns the number of children of this container.
+ *
+ */
+int con_num_children(Con *con);
+
 /**
  * Attaches the given container to the given parent. This happens when moving
  * a container or when inserting a new container at a specific place in the