Roadmap Connection -> Client migration

With new nbxmpp.Client, the Connection class is rewritten from scratch, following things have to be fixed or rewritten

• Fix IBR (Account Wizard)
• Honor SOCKS5 Proxy setting
• Fix ServerInfo Dialog UI
• Test Zeroconf, seems to not work for some reason #10138 (closed)
• Fix Client Cert (needs implementing in nbxmpp) #10139
• Reimplement POSH #10038
• Accounts: Custom Host needs UI for setting type
• Write method to close all account windows on disconnect
• Remove USE_HTTP_ENV var UI (Not needed anymore)
• Correctly shut Gajim down (It tries to send stanzas after we are already disconnected)
• Adapt proxy window (Remove BOSH)
• Accounts: Rework relogin after setting change
• Check Plain Connection Dialog event
• Make LoginD dbus module listen for shutdown instead of sleep #10040 (closed)
• Fix removing account from server
• Fix setting correct resource