A bit of headers cleanup

* Headers in include must contain the structures and prototypes needed
  by the plugins and should be named vlc_*.
* Headers in include/vlc must contain the structures and prototypes
  needed by external libvlc clients

* Moved and renamed some things in headers. 
   - Removed vlc_cpu.h, vlc_error.h (merged in vlc_common)
   - Removed snapshot.h (merged in vlc_vout.h)
   - Removed vlc_spu.h (merged in vlc_osd.h)
   - Removed intf_eject.h and vlc_interaction.h (merged in vlc_interface)
   - Moved all internal headers to src
   - Merged vlc_video.h and video_output.h and move private things to src
   - Removed vlc/intf.h, vlc/aout.h, vlc/vout.h, vlc/decoder.h and vlc/input.h
     (meta headers for use in modules, and often implying too large 
     dependencies)
* Removed some useless dependencies
* Unexported a bunch of functions and structures used only in src/
  (--> Still some work here)
* Finally made input_thread_t and input_source_t (mostly) private. 
  Added input_GetItem to fetch the input_item of an input_thread
* Cleaned up deprecated entries in vlc_symbols.h and bumped up symbol 
  prefix

This commit has a 99% probability of breaking the build, 0.1% of killing
your cat and 0% of getting you hot chicks.