Plugin::preferred does not work well together with PluginParameter

The issue was highlighted by:

https://codereview.qt-project.org/#/c/97761/

Preferred property implies the usage of potentially multiple/different service providers. However each potential plug-in may have different parameters. The parameter names might clash due to different/same naming strategies. A possible solution might be namespacing.

Expected: It should be possible to specify the PluginParameter's for each plugin at the same time.