common: Prepare Quassel::quit() to be used more widely

common: Prepare Quassel::quit() to be used more widely

In the future we want to use Quassel::quit() as the central entry
point for cleanly quitting the application, instead of relying
on QCoreApplication::quit(), which directly stops the event loop
and thus won't process any asynchronous cleanup tasks.

Make the slot public, and protect against multiple invocations.