Filter support for forms

Oh, and it should optionally block the validation of a form (to FORCE the user to reread the changed data)

Finally a fix for the oldest open bug in anewt.new.cxs:
 1463 Sander van Schouwenburg 2009-05-05
      [form] Filter support in form controls.

      You can add filters to AnewtFormControl's:
      1. $control->add_filter($filter);
      2. $control->add_filter($filter, $message);

      The first form silently modifies the value according to the filter. The second
      form checks whether the filter has modified the value, if so blocks validation
      and sets the error message to $message.

Note that filters are still in old anewt style.

Two remarks:

1. I'm not sure whether I like the API (notably the $message argument handling)
2. The validator classes should be reworked :)

In anewt.new.cxs
 1465 Sander van Schouwenburg 2009-07-20
      [filter] Changed filters module to new anewt style.

      Partially fixes #3181.

      Module now called 'filter', base filter now called AnewtFilter, init() changed
      to __constructor() and all filters prefixed with AnewtFilter.

      Also references the form module to Filter changed to the new style.

The code looks quite okay to me. I'll look into merging this (cherry picking, actually) soonish...