Improve revocation dialog

Improvements for all users:

• A lot of desktop clients were disconnected recently. Although it seems the
  issue is not on the app side, users were forced to disconnect, go through the
  onboarding steps again, and synchronize there whole drive back. The latest
  stable release (v3.4.5) allowed selecting a non empty synced directory to
  lower the impact of this issue. This new beta release allows user to exit the
  app from the revocation notification dialog without actually disconnecting
  and try synchronizing again later.

We apologize for the inconvenience.