Document how to create a more interesting VideoFont

Also add "contrib/ges-split-samples.py", an helper script to split
a video file according to labels defined in Audacity.

And add an example list of such a list of labels exported from Audacity,
it's in "contrib/keyboard videofont master C4 B5-samples.txt" and it can
be used to split in samples the example VideoFont at
https://youtu.be/7Btp80LPqRs