IMediaLibrary: Fix onEntryPointAdded event emiting
We use to fire this event only after the folder was added to database & fully processed by the discoverer. This MR replaces the discovery operation by an 'addEntryPoint' and a 'reload' operations.
The addEntryPoint one is prioritized, so any currently running reload will be interrupted, the entry point will be added to database, and the previous reload will resume.
All the operations are still processed by the discoverer thread, since we might need to spawn some fs factories when adding a folder, which we'd rather do from a background thread, rather than from the thread calling IMediaLibrary::discover()
Fix #302 (closed)