Preserve back_and_forth during restart

Add new key "previous_workspace_name" to the json dump of the root container.

src/ipc.c

@@ -651,6 +651,11 @@ void dump_node(yajl_gen gen, struct Con *con, bool inplace_restart) {
         y(integer, con->depth);
     }
 
+    if (inplace_restart && con->type == CT_ROOT && previous_workspace_name) {
+        ystr("previous_workspace_name");
+        ystr(previous_workspace_name);
+    }
+
     y(map_close);
 }