The XML file must begin with:
    <!DOCTYPE Theme PUBLIC "-//VideoLAN//DTD VLC Skins V2.0//EN" "skin.dtd">
    (the "skin.dtd" is ignored but should be an URL on the website)
    and the DTD is looked up at runtime in the directories of the resource
    path (see previous commit), e.g /usr/local/share/vlc/skins2/skin.dtd
    -> the DTD doesn't need any more to be shipped with the themes.