. Added files needed for the forthcoming module management.

 Notes:

   Plugins are now called modules, because we will be able to compile
   them either as a dynamic plugin, or within the program. And, more
   important, I can commit this without having to break the vlc :)

   I tried to be as clear as possible in my comments, please tell me
   if a few prototypes still have an unclear beahaviour.

   Current features:
    . none, files haven't even been added to the Makefile yet.

   Future features:
    . dynamic loading (done)
    . built-in modules (still to do)
    . automatic unloading of plugins (done)
    . dynamic configuration (still to do)
    . automatic choosing of the most appropriate plugin for a given
       task (still to do but well prepared)