Update Locations.xml for Luxembourg: added capitals of cantons and the principal residence of the Grand Duke

Update Locations.xml with the capitals of cantons and the principal residence of the Grand Duke of Luxembourg.

• https://en.wikipedia.org/wiki/Cantons_of_Luxembourg
• https://en.wikipedia.org/wiki/Colmar-Berg

Precised the coordinates of the Luxembourg city:

• https://en.wikipedia.org/wiki/Luxembourg_City