common: Use the Singleton mixin for the Quassel instance

common: Use the Singleton mixin for the Quassel instance

Make use of the newly added Singleton mixon to remove the custom
handling of Quassel's instance pointer. Clearly define Quassel's
lifetime by making it a local instance in main(), rather than
relying on someone to call Quassel::instance() at the right time.

Now it is also clear that the instance is deleted after main()
returns (which was the case before, however there still was a no-op
Quassel::destroy() method too, which has now been removed). This
is fine, because Quassel does not own resources that should be deleted
before exiting the event loop.