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
`::Registration::Storage::Cache.instance.available_addons` should ensure that it never return nil so this code should not be needed otherwise you need it in every place that use it.