Rework layout system to be more flexible
adding "New row" and "New column" buttons, instead of having these as presentation choices
"New row" and "New column" now appear in the document structure. They can also have styles applied to them directly, to allow the entire section to be easily styled.
Ability to have "double width" columns. The HTML/CSS should treat the column as 2 columns, so that the parent div will have something like "columns3" if it contains a normal and a double width column, for example. This should probably be handled by extending the CssClass model with a 'column_equiv' integer attribute.
Reworking the inner mechanism for columns layout. It will start by finding all the columns and rows, creating a column structure, then putting the content into this structure. The corresponding methods that deconstruct a column layout must also be updated