The API should accept provider IDs as service IDs

The OA manifest file allows apps to specify services like this:

{
  services: [
  {
    "provider": "facebook",
    ...
  }
}

but currently the QML and Qt APIs (for example the requestAccess() method) expect a service ID, which would be in the form "myapp.developer_myapp_facebook", which is not very intuitive since these long IDs are not used in the manifest file.

Given that the API doesn't allow an application to use more than one service per provider anyway, we should accept provider IDs as parameters in the API, and just build the full service ID ourselves.