preparser && thumbnailer: change notify behavior when cancelled

Alternative to !6230 (closed) (same first 7 commits)

On top of !6222 (merged)

With this MR, a callback is always called when a request is pushed. This will simplify the integration with libVLC as you won't have to add a new lock and iterate through an internal list when cancelling a request. Indeed, you will be able to get the custom libvlc request object from the data of the Core callbacks.