optionally offer select/autocomplete widget for username filter

Issue Summary

optionally offer select/autocomplete widget for username filter

if there is a settings page for dblog_filters and every filter gets its own tab
well lets use it to provide users with the widget they might prefer

NancyDru has pointed out that a select has more sense (http://drupal.org/node/578488#comment-2124678),
since dblog entries are limited and multiple selection would be allowed

Nevertheless, I prefer more uniformity through every administrative page and therefore I'm used to the autocomplete widget whenever a username is available for selection/filter

Therefore I should provide the alternative to the user's choice.