Add Ability to Exclude Section from Sitemap

Plan:

1. add ShowInSitemap column to Categories table (1 by default)
2. add that column to c and s" units with kOptionFormatter and Yes/No options
3. add that column somewhere on category editing page as checkbox
4. create "sitemap" type (in the "CategoriesEventHandler::getTypeClauses"), that will only return records with ShowInSitemap=1
5. change types="menu" to types="menu,sitemap" to the sitemap.tpl in advanced theme