Use time of creation to weight TM suggestions

Store the time when a record was saved in the TM alongside the
translation.  Use this timestamp to sort results with identical score so
that more recently saved translations take precedence.

This has the highly desirable effect that mistranslations in the TM can
be fixed simply by saving a corrected file.  Because such new records
will have a more recent timestamp, they will be used instead of the
older wrong ones automatically - while still preserving a record of all
alternatives.

Notice that this code handles older TM databases without the "created"
field gracefully: Document::get() returns empty string for fields not
present and stringToTime() translates that into 0, i.e. Epoch, which is
older than any other timestamp.