Allow available features to be probed through libvlc
There are various cases in which a libvlc user might want to know if a feature is available with their libvlc installation.
The original case which brought !2276 (closed) was to know if the podcast module was available, which can be naively achieved by probing for the podcast module, but this is too simplistic for many other modules/features.
We need the probing to be module agnostic, most likely by exposing a known set of features and flagging the modules themselves as handling one or more features in their module descriptor.