preparser + thumbnailer: use an unsigned for the request identification
cf. discussion here: libvlcpp!40 (e7893c16, comment 456278)
TLDR; Leave the request lifecycle handling to the API, the user still could cancel a task if he wants (not mandatory).
To use an ID from libVLC, we have first to adapt the CORE API.
Bonus: preparser and thumbnailer APIs are now more homogeneous.