only save translation for existing strings, require textgroup

There were some bug reports where l10n_client was saving strings in bad textgroups or in other ways added new stuff to the database. Since l10n_client should only translate strings which are already in the database, here is an update which checks for that. One more query for saving strings though, but that is how it is.

Also, this patch makes textgroup required to be non-empty, which should help with bugs like #940142: CCK translation creates strings in the "Built in interface" text group, incompatible with translation_table and other methods again. Tested, so committing this to Drupal 7.