medialib: use new requestThumbnail() API

A new medialibrary API has been added to request asynchronous thumbnail
generation from a media id:
videolan/medialibrary!350



This avoids to retrieve a Media from the media id beforehand via a
blocking call (executing SQL queries) on the UI thread.

Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>