Timetable period heading in graphic timetable viewer (de: Fahrplanperiode als Überschrift im Bildfahrplan)

In some cases additionally to the train heading (see #38 (closed)) a head line showing the current timetable period in the graphic timetable view is available.

Moved here from #36 (closed) (Handling and displaying operating periods): "Optionally displaying as heading in the graphical timetable viewer".

1. Take the chosen train. Additionally other trains will be visualized, but not considered for the heading.
2. Take the first trainPart[@id = train/trainPartSequence[1]/trainPartRef[1]/@ref]
3. Take the referred timetablePeriod[@id = trainPart/@timetablePeriodRef] if available
4. Otherwise (@timetablePeriodRef is absent) leave this line blank
5. Show timetablePeriod/@name if available
6. Otherwise show timetablePeriod/@description if available
7. Otherwise show timetablePeriod/@startDate and timetablePeriod/@endDate if available