remidy API: add annotations to functionos to enforce UI thread, audio thread, and non-audio thread

Currently we do not mark remidy API with annotations like CLAP does or VST3 documentation does, that each function is supposed to be invoked on UI thread, audio thread, or "not on audio thread". 

Probably "asynchronous" as well to indicate that the function leaves immediately, but I'm not sure if it should be applied everywhere.