fix dropdown event listener

fixes middle click in firefox and other issues
see also: https://github.com/twitter/bootstrap/pull/7211#issuecomment-15651878

this fix is currently running on https://zkillboard.com with what seems to be no issues (https://github.com/EVE-KILL/zKillboard/commit/e770a683155f090eb38fd64ea875564080e2c1e0)