Remote administration adapted to new service name (xdm -> display-manager)

- including little refactoring to increase readability and to
  allow easier changes in the future
Changes and version
Fixing a few bugs in my `fix` according to review
Merge pull request #204 from kobliha/enabling_remote_administration

Remote administration adapted to new service name
Switched network service from radio button group into combo. bnc#868256
Updated changelog
Updated help text
Merge pull request #205 from mchf/bnc868256-network-service-switch

Improved network service switching