Move number formatters to the new API

Thanks @pcambra !

There does need to be different classes for integer and decimal 'default' formatters, because they have different default settings, but given the code in the methods, it seems they would really benefit from sharing a common base class.

- At least for the code about prefixes/suffixes in viewElements() should be shared in a method in the parent
(also, beware, in your code this code is wrapped in an if ($this->getSetting('thousand_separator')) { that should be about setting 'prefix_suffix' instead)

- settingsForm() could probably be in the parent as well :
something like :

public function settingsForm(array $form, array &$form_state) {
  $settings = $this->getDefaultSettings();
  $settings_names = drupal_map_assoc(array_keys($settings));

  if (isset($settings_names['decimal_separator']) {
    $element['thousand_separator'] = ...
  }

  if (isset($settings_names['scale']) {
    $element['scale'] = ...
  }
}

(IMO we do want to get rid of the "dummy" settings on number_integer, that are only used for their default values : scale, decimal_separator)

Adjusted the code organisation and bit, and merged into field-plugins-formatters-1785748.
Thanks !