vectors: Implement vector layers from GSoC 2006

Ports the work done by Hendrik Boom, Martin Nordholts, Gilles Rochefort,
and Jacob Boerema to Gimp 2.99/3.0.
8 jobs for !773 with alxsa-vector-layers-2006 in 32 minutes and 24 seconds (queued for 25 seconds)
merge request