Closes #889.

1. Repair identation (my python refused to execute them as they were).
2. Teach normilize_svg to remove explicit "style=fill:" attributes from
elements, controlled by color classes like skin, hair, etc.
3. vector_layer_split moved from inserting vars with sigil for SugarCube
to using "data-transformation" attribute.
4. vector_layer_split output for the 'tw' format became an SVG too, but
with special attributes.

Sync

As discussed in !1887.

Changes to artTools readme: Using SVG normalisation before commiting vector art sources is now mandatory.