Add libvlc download API

Media discovered through libvlc and especially its services discovery modules should be made available to client applications at a bitstream so they can consume it at their own discretion for storage or caching purposes. The client apps shall be able to stop streams at their choice and also be able to recover from a position within the bitstream of their choice. Additionally, they should receive the position within the total duration of the stream and the total size in bytes if applicable.

After previous discussions on the vlc-devel mailing-lists (see https://mailman.videolan.org/pipermail/vlc-devel/2016-May/107867.html), the current suggestion to solve this is covered by !1515 (closed).