Independent source states

### Feature description

So no, it's not about plugin sources voting for their independence and becoming republics.

Currently, the pipeline and the sources and destinations within it always have the same state, be it running, stopped or something else. That also means that if a source malfunctions, the whole pipeline will become degraded, and actually no source will be running.

This issue is about changing that, so that sources have independent states, and make it possible that some sources in a pipeline are running while other sources in the same pipeline are stopped or degraded.

Given some earlier feedback from @lovromazgon and @lyuboxa , this can be complex, but more complex for destinations, which is why we will have a separate issue for those.