Some more note:
After including wicket-{version}.jar file in the WEB-INF/lib folder,
the war file can be deployed without problem. Then it comes another
problem of HTML markup not found.
To avoid that problem, we put back the html to the same place as the
classes files. The webapp runs normally but without CSS formating.
But then there is another more important problem, the embedded jetty
doesn't run correctly with the wicket-{version}.jar file added to the
WEB-INF/lib folder. Somehow it says org/slf4j/LoggerFactory class not
found. Maybe we should only copy the jar file when we are creating the
war file.