Improve layout demo showcase

Improvements to be done:

• align to new theme styling model (maybe re-use the same footer for instance) ? => make it easier to extend an existing theme page so that styles can be reused directly
• add a content view using demo documents
• change global site layout to use layouts and widgets applying to current demo widget type
• try to get rid of needed template + view for each demo widget type, and use a generic one instead
• add all widget types for documentation, not only standard ones
• use converters for specific preview layout needs (like vosabulary selection) instead of adding demo needs in the original widget types definitions.