Forms (and Fields) have no clear method #11
Description
At times it is necessary to disregard any input that has been made before the current time, and there is currently no easy way to do this.
It's fairly easy to conceive a solution which iterates over the Form's Fields, calling each one's widget.clear.
This knock-on effect of this is to require the widget.clear() functionality to be implemented (and added to the API) to all FieldWidgets. Treat this as part of the issue rather than creating an issue for StringFields, ...