Treat stacking containers as if they are in vertical orientation, add testcase

include/con.h

@@ -113,4 +113,12 @@ void con_toggle_fullscreen(Con *con);
  */
 void con_move_to_workspace(Con *con, Con *workspace);
 
+/**
+ * Returns the orientation of the given container (for stacked containers,
+ * vertical orientation is used regardless of the actual orientation of the
+ * container).
+ *
+ */
+int con_orientation(Con *con);
+
 #endif