title: Ask permission to access Camera Roll
body: <p>I have a settings view where the user can choose switch on or off the feature '<em>Export to Camera Roll</em>'</p>
  
  <p>When the user switches it on for the first time (and not when he takes the first picture), I would like the app to ask him the permission to access the camera roll. </p>
  
  <p>I've seen behavior in many app but can't find the way to do it.</p>