Make provider model/dialog dynamic, better sharing

- ServiceProviderModel and LocationModel now connect to the data engine
to automatically update the models on changes,
makes syncWithDataEngine() functions obsolete
- ServiceProviderDataDialog now also connects to the data engine
and gets updated on changes
- Add ServiceProviderDataWidget for more flexibility, used by
ServiceProviderDataDialog, but can now be used without a dialog
- Show provider type and status message in ServiceProviderDataWidget
- Share ServiceProviderModel where possible,
eg. between StopListWidget and StopWidget
- Do not share engine pointers, instead use Plasma::DataEngineManager