Reduce duplicated queries

It doesn't return DISTINCT strings results (#745358: Pager counts destination strings, not source strings). Before other tests and big refator it could simply reduce the duplicate queries.

save queries on its own function & other code execution, eg.

function l10n_community_translate_form(&$form_state, $language, $filters, $strings) {
.....
..
..

  // Fill in string values for the editor.
  foreach ($strings as $string) {
    $form['strings'][$string->sid] = _l10n_community_translate_form_item($form_state, $string, $language, $filters);
  }

  return $form;
}