When having a hash-of-hashes where each hash is indexed by a name,
(such as GKeyFile), you can either implement it as a hash-of-hashes
or define your own version of indexes that pack both levels of names
into one key.

This is an implementation of such a key. Use it as:

  GHashTable *hash = g_hash_table_new_full (_nm_utils_strstrdictkey_hash,
                                            _nm_utils_strstrdictkey_equal,
                                            g_free, _destroy_value);

and create keys via:

  NMUtilsStrStrDictKey *k = _nm_utils_strstrdictkey_create (s1, s2);

For lookup you can use static strings (note that the static string
might increase the size of the binary):

  g_hash_table_contains (hash, _nm_utils_strstrdictkey_static ("outer", "inner"))