Instead of relying on the QmlPreviewRunWorkerFactory to match for all
RunConfiguration, every plugin needs to create a WorkerFactory for
its own RunConfiguration.
Similar to the SimpleTargetRunnerFactory there is now a
SimpleQmlPreviewRunnerFactory which makes the setup easy.
Change-Id: I350e5a047033a259dae91045486d4f41e7f58994
Reviewed-by: hjk <hjk@qt.io>