Add network persistence

[ERnsTL]

• in order to not having to build up the network again after restart ;-)
• "never lose your data"
• TODO where is the button in noflo-ui to trigger persistence?
• TODO automatic saving in time intervals?
• TODO integrity checker
  • critical and non-critical errors
  • critical - cannot load, cannot start network
  • noncritical - found missing connections, unconnected ports, unavailable components.
• TODO keep .bak file of previous version
• TODO saving on ctrl-c? No, ctrl-c means "abort".