Add styling for the layout demo

The layout demo needs some styling. It uses the theme "galaxy/popup" for its rendering as it shows layouts as they would be in the default application.
But the application is standalone so it does not use the theme for its specific styling: it has to be done in the file src/main/resources/nuxeo.war/llayoutDemo/style/layoutDemo.css

Main work is about:

• styling the "menu" and "show sources" foldable boxes and lists to make them different (use a different color for instance)
• adding an icon in the header
• maybe add a footer