[Qt] allow setting listen via GUI

- add DEFAULT_LISTEN in net.h and use in the code (shared
  setting between core and GUI)

Important: This makes it obvious, that we need to re-think the
settings/options handling, as GUI settings are processed before
any parameter-interaction (which is mostly important for network
stuff) in AppInit2()!
Merge pull request #4290

56b07d2 [Qt] allow setting listen via GUI (Philip Kaufmann)
small cleanup of net

- remove an unneded else in ConnectNode()
- make 0 a double and change to 0.0 in ConnectNode()
- rename strDest to pszDest in OpenNetworkConnection()
- remove an unneded call to our REF() macro in BindListenPort()
- small style cleanups and removal of unneeded new-lines
convert an if into an else if in OpenNetworkConnection()
Merge pull request #4227

634bd61 convert an if into an else if in OpenNetworkConnection() (Philip Kaufmann)
5bd6c31 small cleanup of net (Philip Kaufmann)
[Qt] rework ClientModel::getNetworkName()

- also use it in paymentserver for getting the network name