Read list of translated languages from a file

Read list of translated languages from a file

The previous scheme of loading all possible translations and checking whether the work
is a bit too much "brute force" and causes problems on Mac OS X (documents loaded
with the wrong language).

In the new scheme, autotools install a file lib/installed_translations that contains a list of installed languages (the .gmo files that got installed). This file is read
in Languages::readInstalledTranslations and allows to set the translated() property
of each language.