Rework discovery & parsing callbacks

This aims at fixing #339 (closed) and improving the feedback available for the applications using the media library.

It changes:

• The way we provide updates about the discovery in progress. Instead of a onDiscoveryStarted/onDiscoveryCompleted per entrypoint, users will now receive a single onDiscoveryStarted/onDiscoveryCompleted invocations per group of tasks. onDiscoveryCompleted will now be invoked when the discoverer has completed all its jobs.
• No changes to onEntryPoint* callbacks, they are still emited asynchronously after a ban/unban/remove tasks gets executed
• Discovery errors are now propagated through a dedicated callback (onDiscoveryFailed)
• Parser progress is now providing its number of tasks completed & scheduled, in order to let the application have more control about how it displays the progress. This will also update more often when processing large number of items, while the callback was only invoked upon a change in the provided percentage

This is still an open discussion and comments/opinion are more than welcomed