Add support for type conversion in xwork.xml

The DTD specifies a "converter" attribute for action. In ActionConfig comments it is also mentioned in comments as converter name. However the support is not working.

A nice improvement would be to be able to specify custom TypeConverter per action in the xml config, and specify default converter to be used (in webwork.properties, or in xwork.xml, like the tag existing for default interceptor stack)

I'd suggest also moving the support for type conversion from properties files to xwork.xml (as sometimes having to maintain i18n bundles, validation xml file, action java code, views code and converter properties starts to be too many),while the experience shows, that with each change xwork.xml tends to be getting more concise and readable (at least IMO)