Handle #xwikicontent as a column in the layout grid

In order to be able to nest new layout structures.

Thanks lucaa for the help with this issue. You can read more at "About using bootstrap/flamingo for customizations in XWiki" http://xwiki.markmail.org/thread/jkpunp2aynrffpba

Read more about grids and Bootstrap at http://getbootstrap.com/css/#grid-nesting