Allow network media to be cached locally

When displaying some media coming from a network storage, it's highly likely that the users will want to cache these media locally on their device if they move away from that network storage, for instance a user transiting to work wants to cache a specific album for their journey

Most likely we want to be able to monitor the cache folder without exposing it to the end user. This is only needed if we need to react to an external user deletion, and we could just rely on the application to invalidate the cache if a file isn't available in the cache anymore.

Required features:

• Listing all cached files from the database
• Invalidate a given media cached version
• Invalidate the entire cache
• Add a helper to fetch the main file (in term of what the application will ask libvlc to play) which favors the cache version

To be determined:

• Should the ML handle the caching itself, or should libvlc expose an API to do so
• Specify the cache settings: max size, max number of files, a max size per container (for instance 1GB for caching a NAS, 250MB for a podcast A, ...)