Add HTML5 attributes to text widgets.

With this, textarea, textinput and password templates support the
*maxlength*, *placeholder* and *required* attributes.

- *required* already exists as a field attribute so we just read that.
- *maxlength* and *placeholder* are read from the widget instance.

Also, HTML5 supports several new *type* values ('date', 'number' etc.)
so this becomes flexible in textinput.pt so this template can be reused.
It reads widget.type and defaults to "text".

What about people who aren't using HTML 5?
==========================================

Why, I don't know, are there any?

Maybe it is better if *you* tell me how to proceed. Should Deform be globally
informed about the HTML version, then templates can be adjusted? How would we
do this?