Improved description for landscapes, same as for skycultures

Currently (0.10.4), landscapes are described only in a single paragraph in the landscape.ini file. It would be fine to allow an optional description.<language>.utf8 file to better explain possible interesting features in the landscape, with additional photos etc., and hyperlinks, similar to the example in the aztec skyculture. In absence of this file, the description= paragraph in landscape.ini shall be used.