correctly sort numbered workspaces (+testcase)

Numbered workspaces (workspaces with a name containing only digits) will be
inserted in the correct order now. Named workspaces are always sorted after
numbered workspaces and in the order of creation.

src/ipc.c

@@ -269,7 +269,9 @@ IPC_HANDLER(get_workspaces) {
             y(map_open);
 
             ystr("num");
-            y(integer, con_num_children(ws));
+            if (ws->num == -1)
+                y(null);
+            else y(integer, ws->num);
 
             ystr("name");
             ystr(ws->name);