Add a "decorative" or "container" widget type

Sometimes layouts require more presentation features, that could be addressed us-
ing "decorative widgets". These widgets would not map any data, but would be
helpful to either:

• separate forms semantically
• add titles to areas semantically close.

This has already been addressed in Studio (with widgets holding a property
"header") and other client projects (with custom templating, or with widgets displaying an
"<hr />" tag.

The "title" or "header" decorative widgets could also be defined using a simple
"text" widget with a hardcoded value to bind.

This is currently achieved writing specific "<h3>" tags in the layout template, and
using the layout or widget name to display the title.
When the area needs to be closed (borders defined using CSS), it also needs a
"<div>" tag all around the widgets to achieve this rendering.

TODO: define syntax for declaring these widgets and decide if bare HTML
code should be accepted for them