translators are installed via BTranslatorRoster::StartWatching().
* Fixed GetTranslatorData() - some hooks are only optional (this puts the
JPEG, GIF, and some other translators back into the game).
* Made BTranslatorRoster::Default() thread-safe.
* Improved documentation.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@17267 a95241bf-73f2-0310-859d-f6bbb57e9c96
