Fix field variables resolution in sub widget templates

Steps to reproduce:

• define a complex property, mutlivalued, with 2 different fields
• define default widgets for this property, but change the complex property subwidgets to use a widget template
  The widget template maps the 2 fields and references them using # {field_0} and #{field_1} as usual
  
  => the layout cannot be submitted correctly and crashes with an error like:
  
  Caused by: javax.el.PropertyNotFoundException: /widgets/2fields_widget_template.xhtml @17,63 value="#{field_0}

  ": /widgets/complex_widget_template.xhtml @84,67 value="#

  {field['firstName']}": Target Unreachable, identifier 'field' resolved to null
  at com.sun.facelets.el.TagValueExpression.getType(TagValueExpression.java:62)
  
  or if the sub field value was already set:
  
  Caused by: javax.el.ELException: /widgets/2fields_widget_template.xhtml: /widgets/complex_widget_template.xhtml @84,67 value="#{field['firstName']}

  ": Property 'firstName' not found on type java.lang.String
  at com.sun.facelets.el.TagValueExpression.getType(TagValueExpression.java:62)

This makes it impossible to use widget templates for sub fields.