Desktop: Seamless-Updates: triggering updates

[AliceHincu]

This PR triggers the updates.

• Automatic check:
  • if an update is available, show a notification:
    
  • if an update is not available, do nothing
• Manual check (from toolbar choose Help > check for updates):
  • if an update is available, show the same notification
  • if not, the user is notified that no updates are available:
    

If the user chooses to update later, then the updates will not be triggered (not even if the app is closed). If the user chooses to restart now, a new version is installed (the old one is automatically deleted).