Add new Client class
This MR uses the new nbxmpp.Client
This has the benefit that most of the complexity of connecting to a server (Resolving DNS, choosing transport/connection type) is now in nbxmpp and not anymore in Gajim
Because this is a big change to Gajim core some things are expected to break. A list of things we will fix in the near future is here #9987 (closed)