Region preview should be generated by each module

Currently the region preview is generic code which does different things depending on the module. This is not future proof as new modules will have no logic in there to display themselves - also schema changes on the module will render the code that is there out of date.

The suggestion is that each module implements a Preview() function which takes the width and height of the area it is in and returns the HTML for displaying that region. The layout preview would then just create a module object and call Preview(width, height).