Add Workflow->defaultSettings() from interface PluginSettingsInterface

Problem/Motivation

Please add the get/set Settings functions from interface PluginSettingsInterface.
It makes the code more stable and shorter. And allows easiers maintanence when adding settings.

API changes

WorkflowInterface then implements:
• public static function defaultSettings();
• public function getSettings();
• public function getSetting($key);
• public function setSettings(array $settings);
• public function setSetting($key, $value);