building-comparison: improvements to figure

[matthiasschaub]

• display absolute area in sqkm above or in bars
• add information about references datasets (maybe not necessary because of provided links?)
• Separate description text
  • order by reference dataset
  • first, warnings
  • then (separated by new line), description
  • continue with next reference dataset
• improve result description
  • 100% of the area-of-interest (Saxony-Anhalt) is covered by te reference dataset Eubucco. The building area of OSM is 95,47% of that reference data and is therefore classified as „high complete