title: UWP: Detect app gaining/losing focus
body: <p>I want to be able to prevent the screen saver from triggering while my app is in use by using the DisplayRequest class, but I only want to do this while it's the active app. If the user switches to another window/app then I want to act like a good citizen and allow the screensaver again.</p>
  
  <p>I can't see an obvious way to detect when a UWP app gets/loses focus (or is activated/deactivated) and a quick search around doesn't offer any insights. Can anyone point me in the right direction?</p>