Clean img directory from nuxeo-webapp

The content of the nuxeo-platform-webapp/src/main/resources/web/nuxeo.war/img is not relevant:

• unkown resources (cnt.bg.gif, bg.gif)
• icons (core_servers.gif, core_servers.png, ImportContinue.png)
• old resources (rightsadd_bar.gif, syndication_bar.gif, syndication_button.gif)
• old folders: theme_galaxy/

To do:

• check what is still used
• (re)move unused resources to nuxeo-resources-compat module
• move icons in an icons folder

+ remove old resources from the nuxeo error pages