FR: Add getChangedValues() method

[milo]

Can be useful to receive only updated values from form/container. For example:

$diff = $form->getUpdatedValues();
if (count($diff)) {
   $db->update(...);
} else {
   $this->flash('Nothing changed.');
}

# or
$this->log("User $username:", $form->getUpdatedValues());

API changes from top of my head:

• FormContainer::getChangedValues()
• BaseControl::setAlwaysChanged(bool ...)

Hard to decide in which point create an original values snapshot. Probably exactly before Form::getHttpData().