Add initial draft for background operation (loading the state and activate the plugin from a background thread)

[abique]

The idea behind this design is to let the host run some expensive operation on a background thread to reduce the load on the main thread.

I've chosen an approach with a clear transition between the main thread to the background thread bgop-thread, because I think it is easier to work that way rather than having to be extra defensive everywhere and expect a call to happen at anytime from a random thread.