Add an API to request asynchronous thumbnail generation from a media id.
The existing API (IMedia::requestThumbnail()) needs a Media instance. If the client only knows the media id, it must execute a SQL query to retrieve a Media, so for an UI client, this defeated the purpose of an asynchronous API.
This new API resolves the media id from the worker thread.
Refs #283 (closed)
VideoLAN code repository instance