Move-overwrite File & TOS Update Dialog

Improvements for all users:

• Moving a file to overwrite an existing one is now effectively synchronized
  as expected.
• Users don't look at the systray popover so often. So people who have not
  accepted the new TOS may be unaware that the synchronization is currently
  paused. The app now shows up a reminder dialog on start when appropriate.

See also known issues.

Happy syncing!