Needs to handle network connects/disconnetcs

When chatty is started without a network connection there's no indication in the UI just:

sm.puri.Chatty:9325): chatty-connection-DEBUG: 09:19:17.644: Disconnected: "agx@sigxcpu.org/bogon" (prpl-jabber)
  >Error: 0
  >Reason: Unable to connect

in the logs. When the network comes up later chatty does not connect to the XMPP server so there's several issues here (some of these might need design) but some are simple:

• when the network comes online try to connect to XMPP accounts (see e.g. https://gitlab.gnome.org/GNOME/krb5-auth-dialog/blob/master/src/ka-kerberos.c#L1097 how to detect if the there's a network connection)
• when chatty is started without a network connection should we ask the user to open one (prefereable) ? This likely needs design for the overall case (in chatty and s.th. like g-s-d) or should we just show an error in chatty for the moment? Since this also affects other apps it needs design @tobias.bernard ?
• similar when the network connectiong goes down (not initiated by the user).

The last two points affect all apps with internet connection, taking chatty as an example since it's furthest along on the phone atm. Let me know when we should split issues.