Improve support for portlet integration in velocity templates

Modify the velocity templates to improve the support for portlet integration:

• don't display the left/right edit/view panels in portlet mode
• use a different container (div rather than body) for XWiki output in portlet mode but with the same CSS classes
• load the style sheet extensions in portlet mode (the generated HTML is not clean currently but the SSX plugin could be changed to take into account the portlet mode and use style element with @import instead of the link element)
• avoid URL literals in JavaScript code (e.g. when registering shortcut keys)