Display a standalone widget in a given mode

Widgets can already be registered independently from layouts. This makes it pos-
sible to reuse a defined widget in several layouts.

Mechanisms to render a widget independently from a layout already exist: the tag
"nxl:widgetType" does render a widget, but needs the whole widget configuration
as attributes, and the tag "nxl:widget" performs rendering of a given widget instance, that has been already computed by the parent layout.

The nxl:widget tag could be improved to take only the name of a global widget, a mode, the
object to map, and query the layout service to get the configuration. That kind of
tag could be used in every page, inside or outside a form. As a reminder, layouts
and widgets do not require a document model: any kind of object will do, using the
object attribute name (getter/setters need to be available) as field names.

As an alternative, providing a widget definition should also be made possible, to allow dynamic creation of widgets to render.
This can be useful to render widgets referenced by actions for instance.