Ui::SslErrorDialog belongs within the Mirall namespace
Make "Sync started" message optional

Fixes #934
Try to be more graceful during shutdown

- Do not delete _tray, use deleteLater() via QScropedPointer
- force closing any dialoges that might want to write their
  geometry in aboutToQuit(), before the actual destruction
  of the sync thread, etc starts.

This tries to fix #945