xembed-sni-proxy: Add Menu property

Because we did not have this property the adaptor used a default constructed empty path. An empty path is invalid though and Qt would refuse to marshall it. Unconditionally advertise '/' as the menu path even though we don't have one. Callers looking for a menu there will receive an error that there's not the interface that they expected at the path. BUG:439229 FIXED-IN:5.22.3