Fix include of layout tags sub tags

Layout widget tag handlers API makes it possible to take sub facelet handlers into account, but they're not correctly propagated during the layout/widget construction.

This is useful to be able to add custom behaviour to widgets (ajax behaviour for instance) from the layout template or other templates like for instance:

<nxl:widget widget="#{widget}" value="#{value}">
  <a4j:support [...] />
</nxl:widget>

or

<nxl:widgetType name="template"
  template="/widgets/summary/action_link_widget_template.xhtml"
  mode="view" value="#{action}" field="">
  <ui:define name="inside_action">
    yo
  </ui:define>
</nxl:widgetType>