qtui: Fix quit sequence and lifetime issues

Make use of the Singleton mixin for Client and GraphicalUi. Fix
ownership and explicitly construct and destroy the pseudo singletons.
Remove some static init methods in favor of initializing in the
constructor where possible. We still need delayed initialization
of the main UI however, so GraphicalUi::init() has to stay.

Simply calling QCoreApplication::quit() doesn't clean up properly,
because it immediately stops event processing, which means that
any deferred deletions won't be processed anymore.

Since we perform some important and asynchronous tasks during
shutdown, such as saving settings and properly unregistering from
IRC, we really want proper cleanup, though.

Fix this by relying on Quassel::quit() instead, and registering
appropriate quit handlers that shut down the client in orderly
and controlled fashion. Ensure that all open windows and dialogs
are closed prior to MainWin's destruction.

Core shutdown warrants a more complex approach, which will be
implemented in a follow-up commit.