ControlServer does not broadcast preview changes

[Kunsi]

When selecting a new source in voctogui without hitting a transition button, the selection is not broadcast to other connected clients. The clients only get notified when the transition is done, making it impossible to implement preview into a tally light solution.

[Kunsi]

Just took a look into this. If we change some OkResponse to NotifyResponse, the change of sources correctly gets broadcast to the clients. However, the protocol currently has no method of broadcasting a planned composite mode change to the clients.

This means we have to create a new status command to voctocore to be able to do this. (The alternative would be to fix the best command to include the composite mode, which would be a breaking change and thus require us to release 3.0)