Fix required property evaluation in widget templates

Sometimes, the widget "required" property is used referencing expression

#{widget.required}

instead of

#{widgetProperty_required}

The first expression is only a shortcut to be able to know easily whether the widget should be displayed as required in templates, but this expression is resolved only once, at build time.

To ensure good propagation and evaluation of this property in all circumstances (especially when using expressions depending on the request attributes), the second expression should be used instead.