General idea:

                            /     gstv4lsrc.[ch]
                       /          v4lsrc_calls.[ch]
                    /
gstv4lelement.[ch] -------------  gstv4lmjpegsrc.[ch]
v4l_calls.[ch]     -------------  v4lmjpegsrc_calls.[ch]
                    \
                       \          gstv4lmjpegsink.[ch]
                            \     v4lmjpegsink_calls.[ch]


I.e., all the files on thei right are child classes of
the v4lelement 'parent' on the left.mjpegsink is still
todo.

Generic idea:
* v4lelement handles generic v4l stuff (picture settings,
    audio, norm/input setting, open()/close())
* v4lsrc, v4lmjpegsrc handle the capture specific
    functions. Maybe we'd need a v4lmpegsrc too
* v4lmjpegsink handles mjpeg hardware playback of video