fix support of audio channel mixers, like the headphone channel mixer
    for instance, which was broken since the introduction of user audio
    filters; if specified by the user and compatible with the audio
    pipeline, we add the channel mixer after the user filters.
    -> unfortunately, this fix removed two strings and introduced 4 new
       ones
    -> TODO: allow the user to choose only amongst channel mixers, not
       all available audio filters

* vlc.exe.manifest Makefile.am install-win32 :
    add a manifest file to allow the win32 gui to use Windows XP visual
    styles when available.