Add a way to delay the loading of the backend plugins

At the moment the plugins are loaded and connected once the QML component is complete. As this doesn't mean the whole QML tree is created yet loading the backends can have some bad effects to the startup time.

This task is about adding a new way to delay the backend loading to a later time. E.g. by using invokeMethod to start the autodiscovery not right away, but in the next event loop run.