LIBKML: add layer creation option FOLDER to optionally write layers as Folder instead of Document

git-svn-id: https://svn.osgeo.org/gdal/trunk@27073 f0d54148-0727-0410-94bb-9a71ac55c965
Fix comment

git-svn-id: https://svn.osgeo.org/gdal/trunk@27074 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx