Support for clearing more form input elements

[labkey-jeckels]

Rationale

It's handy to clear values from more than just file <input> elements

Changes

• Move clear() method up the hierarchy

[labkey-danield]

I suspect the clear was done by calling set("")

[labkey-jeckels]

I suspect the clear was done by calling set("")

That's what I tried initially (and then with null) but both failed trying to parse it as a date. I didn't save the stack trace.