New connect api

Includes some fixes/enhancements:

-  SLP selection dialog: make OK button default, display the default SCC URL
- Do not set repository status (enabled/autorefresh) for service repositories, libzypp has been fixed to set correctly the status from the service (libzypp >= 14.21.0)
- Fixed credentials file removal (File.unlink is correct)
- Do not use long and short extension name together
ha good catch, that factory should in fact allow explicit nil