Add displayPropertiesFile support for selenium:selenese goal

We're trying to use the xvfb goal in conjunction with the selenese goal for running headless HTML test suites. It appears that the selenese mojo doesn't support the displayPropertiesFile configuration element which would allow it to pickup the DISPLAY environment variable correctly from the xvfb goal.

We can work around it by manually setting DISPLAY to :20 (or other specified for xvfb). But it would be nice if the selenese goal had similar support for the properties file, as selenium:start-server does.

Thanks!