Merge pull request #393 from apoloval/task/port-fw

Several changes to implement port forwarding in message gateway
Detect external IP for port forwarding connections
Remove coinffeine.externalHostname

It was unnecessary, as the external ip is detected.
Merge pull request #394 from sortega/detect_external_ip

Detect external ip
Add a new tab to preferences view for network configuration
Move some common stuff to `PreferencesTab`

Move `closable` setting and require a tab name in child classes.
Several improvements on preferences form

* Improve the OK Pay tab and adapt it to the new sizing
* Use the term `apply` rather than `close` referring to the preferences
* Fix some typos.
* Remove magic numbers
Merge pull request #395 from apoloval/task/settings-improved

Add a new tab to settings form to configure the network