Add support for Log4J2 as a struts and xwork logging backend

Struts2 has it own logging layer which can be configured to use commons, jdk or slf4j as a logging backend.

Log4J2 provides similar to Slf4J an own logging api.

A new Log4J2 implementation should be enabled in the same way like the Slf4J logger implementation.

{{
-Dxwork.loggerFactory=com.opensymphony.xwork2.util.logging.log4j2.Log4j2LoggerFactory
}}