Refactoring SVGxxxList for SMIL animation

Craig Topper is working on SVGLengthList and we've been discussing the issues.
Turns out this is a "non-trivial" project (oh wait, we knew that), so I've made
a start on a wiki page to help nail things down and hammer them out. Anyone
that's interested in the topic is encouraged to read it through and add your
thoughts. I'm particularly interested in input from the SMIL guys about the
animation assumptions.

https://wiki.mozilla.org/SVG:Lists

Jonathan