[INTERNAL] made stun port optional

STUN port can now be left blank in the NetworkPreferencePage. If it is
not set the default STUN port will be used.

Change-Id: Icb6613fd600d644b1f8fe71a7fcbc508c7481395
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1132
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>