Add support for XML properties files.
When building internationalized sites, properties files become a pain to use.
".properties" files are encoded using ISO 8859-1 so characters not in the
encoding have to be written using Unicode escapes.
The native-to-ASCII Converter can be used, but much better approach would be to
use XML properties files. Since UTF-8 encoding can be used, there is no longer
the need to use Unicode escapes.
Can org.apache.struts.util.PropertyMessageResources be modified to load ".xml"
if the ".properties" does not exist.