Illustrations in sky culture descriptions

At the moment, there can't be any illustrations in sky culture description files, because sky culture folders have different locations on the different platforms. Moreover, a sky culture directory can be found in two different places - in the installation directory and in the user data directory. This makes using static relative paths ineffective.

A possible solution is to have a "variable" (e.g. $SKY_CULTURE_DIR) in the description file that will be replaced in runtime with the automatically detected path to the sky culture directory before the file is displayed.