preparser: add GenerateThumbnailToFiles API

This will avoid to parse/decode the same video more than one times when generating more than one thumbnail from one video (when generating thumbnails with different sizes, like it is already done by the medialibrary).

This will be particularly useful when using the preparser via an external process. Indeed, it will avoid passing the generated thumbnail via IPC/json. It will even be faster than the current system, since a video will now be decoded only one time for thumbnailing.