look and feel: create split container when switching workspace layout

Quote from the source:
  When the container type is CT_WORKSPACE, the user wants to change the
  whole workspace into stacked/tabbed mode. To do this and still allow
  intuitive operations (like level-up and then opening a new window), we
  need to create a new split container. */

include/con.h

@@ -153,4 +153,12 @@ Rect con_border_style_rect(Con *con);
  */
 int con_border_style(Con *con);
 
+/**
+ * This function changes the layout of a given container. Use it to handle
+ * special cases like changing a whole workspace to stacked/tabbed (creates a
+ * new split container before).
+ *
+ */
+void con_set_layout(Con *con, int layout);
+
 #endif