Read port-forwardings from settings file

[JayFoxRox]

This is a (hacky) partial implementation of #1.

• The UI is read-only for now; settings have to be made in settings.json (manually).
• It only implements port-forwarding, and not the socket networking that would be more useful for l2tunnel (although I'd expect a lot of code to overlap).
• It doesn't bind widgets to settings like the rest of the code does.
• It uses a structured settings section (this could be improved).
• It does not support port-ranges (most routers would support this).
• It does not support multi-protocol port-forwardings (some routers have a "Any", "Both" or "UDP/TCP" option).
• Hackily calls iterator elements e; please suggest better names.
• Is currently active, regardless of "Cable Connected" setting (might lead to security issue as user is not aware their network is exposed).
• I disable the port list because it's read-only (enabling it allows editing with our current code and UI file). However, disabling it also disables the scrollbar, so you can't scoll (if too many ports). Can be worked around by changing window size.

The configuration looks like this:

  "network": {
    "port_forwarding": [
      {
        "name": "Xbox traffic",
        "protocol": "udp",
        "guest_port": 3071,
        "host_port": 3071
      },
      {
        "name": "FTP",
        "protocol": "tcp",
        "guest_port": 21,
        "host_port": 8021
      }
    ]
  }

(Xbox traffic is actually on port 3074 - ignore my mistake in the example ports)