modules/misc/lua: Share some code between the modules that uses lua and put them in a single module.

Fixes #882

control/media_descriptor.c: Keep a media_list of our own subitems if we have some (we are then a media_descriptor playlist).

- Factorize some code

input/meta.c: Make sure we don't return VLC_SUCCESS in __input_FindArtInCache when no album art was found.

(Oh, I am well aware that this is often not correctly set, but then again,
 why the heck would someone wants to parse the SDP origin in real life?)