Change the behavior of the field overview form.

 (1) there is a new "add new field" section that will create a new field for the structfieldstorage field storage type
 (2) if the default field storage system is not structfieldstorage, then we add a section for these fields creation as well
 (3) we only display the field type and widget types that are known to be working with structFieldStorage when that field storage system is being used

Overall this update gives more flexibility to the creation and management of the content types per field storage system!