Improve CSS and scripts resources management in OpenSocial gadgets

This is needed to easily override the styles or content of the Nuxeo OpenSocial gadgets without redifining the whole gadget.

• Move styles from XML to external CSS files
• Move scripts from XML to external JS files
• Move CSS and JS files as static theme resources in nuxeo.war
• Update the gadgets to use the new resources

This will allow developers to easily override the CSS or JS files they want.