Debugger: Do not directly call Engine::notify() functions from other threads

We call updateViews() and similar and therefore need to be in the gui thread.

Change-Id: Ia0e9be59dbcc529341449ba7f64a93cda320ff5b
Reviewed-by: Ulf Hermann <ulf.hermann@digia.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>