title: Magento: Flush widget from block / fullpage cache if widget parameters change
body: <p>We have a widget on the start page of our shop.</p>
  
  <p>The shop uses FPC and block cache.</p>
  
  <p>How can we update the widget contents if the admin changes the widget settings in the backend?</p>
  
  <p>Is it possible to use the Widget's configuration data in the CacheKey?</p>
  
  <p>Or do we have to set the cache lifetime very small?</p>
  
  <p>EDIT: My own answer uses the cache lifetime. 
  What would be the way to pointedly clean the cached blocks from FPC only when the widget instance is edited?</p>