Tidy up JClient plugin interface

Tidy up the plugin interface so that all calls are provided by the Media superclass - eg:
log(int, String) instead of DisplayClient.debug.log(int, String)
redrawBackground() instead of parent.redrawBackgroud()

Also methods documented as final in http://labs.xibo.org.uk/blueprints/Home/documentation/plugins need to actually be made final.

Also scopes need to be tightened on some methods/variables.