Implement tree flattening to automatically solve situations of redundant chains of split containers

This should fix the move problems. See comment of tree_flatten() for a little
example.

src/con.c

@@ -669,6 +669,8 @@ void con_set_layout(Con *con, int layout) {
         if (old_focused)
             con_focus(old_focused);
 
+        tree_flatten(croot);
+
         return;
     }