correct attribution / gpl

[adam-ce]

1. we need correct attribution to basemap, i.e., https://cdn.basemap.at/basemapat_Copyright.pdf
2. we need a link to the source code and the full GPLv3 license, i.e., see https://fossa.com/blog/open-source-software-licenses-101-gpl-v3/ (alpinemaps.org is not conformant atm).