[Seaside] Nested form tags

[Michel Bany]

>     That's true, but this lets us take more appropriate action - either
>     throwing a meaningful error, or possibly just ignoring the inner form
>     and treating it all as one big one (which, I think, is my
>     preference).  So I'm very grateful that Michel took the time to
>     figure this out... 
>
>
> I think I'd prefer if we had an #ensureForm:  version, that would just 
> submit the one big form, and still have #form: around, which would 
> error out when nesting; this might capture some mistaken composition..
>
I am currently trying something for which I would like to get opinions :

WAHtmlRenderer and WARenderCanvas will have two new methods :
    #shouldRaiseNestedFormErrors
    #shouldSilentlyDiscardNestedForms
both methods expected to answer a boolean.

The current Seaside behavior would be that both methods would answer false.
However I think that the default implementation could be for the second 
method
to answer true, this would satisfy Avi's preference.

Those who are not happy with the default implementation would have the 
possibility
to subclass and re-implement as needed.