ECW: add capability to set/alter/unset the FILE_METADATA_xxx metadata items contained in an existing ECW v3 file (with gdal_edit.py for example)

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@25858 f0d54148-0727-0410-94bb-9a71ac55c965
Fix compiler warning

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@25859 f0d54148-0727-0410-94bb-9a71ac55c965