Merge branch introducing wrapper groups
    
    Browser storage 2.1 does not store the wrappers directly. Instead all
    wrappers are grouped.
    
    This deals with:
    * Redundancy: the levels define a group instead of all the parts of the
      group. We already had inconsistences when not all wrappers were part
      of all levels.
    * Future extensibility: if a new wrapper is included in a group, all the
      built-in and user-defined levels will be updated automatically.
    * The option page code is simplified and both HTML and settings based on
      the HTML are now defined automatically.