Run preparser tasks in a separate process

As in discussion here and multiple off-record discussion since the integration of the medialibrary:

https://mailman.videolan.org/pipermail/vlc-devel/2020-September/137519.html

Preprocess tasks must be done in a separate processus so as to avoid explosion of VLC in the background without user execution. It allows a nicer way to blacklist the failing items.

The preparser object should probably stay in the interface process, but every preparsing subtask must be executed in the other process.

It should probably support running in the same thread too though, for debugging and easing the creation of ports to new systems which might not have the process integration in VLC yet or a different kind of process execution (think WASM platform for example).

This property also allows the implementation of one platform before integrating the other, thus enabling a more incremental approach.