GML: add support for parsing .xsd with a <choice> of polygonProperty and multiPolygonProperty

git-svn-id: https://svn.osgeo.org/gdal/trunk@28066 f0d54148-0727-0410-94bb-9a71ac55c965
Fix sample so that it validates

git-svn-id: https://svn.osgeo.org/gdal/trunk@28067 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_vc12