frei0r-plugins: Move OpenCV plugins to a subpackage
    
    For the F19 live DVD we have cheese -> gnome-video-effects
    ->frei0r-plugins -> opencv, the last being 31M on disk (20 of which,
    appallingly, is XML).  opencv is only used for the face-detection
    plugins in frei0r.  The only other consumer of frei0r-plugins in the OS
    is pitivi, and neither it nor g-v-e uses the face-detection plugins.
    
    Move them to a subpackage to break the dep chain.
    
    Signed-off-by: Adam Jackson <ajax@redhat.com>