Provide a special `<widget>`-Tag on upper-level HTML templates

[phorward]

A special <widget>-Tag could be used to identify the current html5.Widget-subclass.

class MyText(html5.Span):
    def __init__(self):
        super().__init__(
            """
            <widget class="big" title="Hahaha">
                Hello My Text
            </widget>
            """

would result in

<span class="big" title="Hahaha">
    Hello My Text
</span>

The special <widget>-Tag should only be allowed at top-level of the Template.