FileSystemLayer: use XDG_CACHE_HOME as base path on Linux
FileSystemLayer: use XDG_CACHE_HOME as base path on Linux

fixes #57. gives us a writable path when run as snap.
paroj/ogre,paroj/ogre,OGRECave/ogre,OGRECave/ogre,OGRECave/ogre,paroj/ogre,paroj/ogre,paroj/ogre,OGRECave/ogre,OGRECave/ogre