Application global d-bus connection remains invalid for app lifetime if created before d-bus daemon is running

If application calls QDBusConnection::systemBus() function before d-bus daemon is running, it will create invalid global d-bus connection which remains invalid for the lifetime of application. So at creation time of global d-bus connection there is no check if creation actually was success.

Steps to reproduce (use provided test app):

1. Shut down all d-bus daemons (main, socket, session)
2. Start application, it should print out dbus connection nok
3. Start d-bus daemons
4. From test app UI click all buttons.
5. Observe print outs in console

Expection would be that all should print out ok. Now only "manual" will print ok as for working d-bus connection.