Add support for Thunderforest maps

[sgelb]

https://www.thunderforest.com provides some nice maps, e.g. OpenCycleMap or Landscape. An especially useful feature is the use of bilangual labels (see #331) on most of their maps.

Implementation into osmdroid should be pretty forward, but as the service requires an account and an api key per user, it would be neccessary to get and save these infos per user.