3D viewer: simplify exported SVG by extracting styles
3D viewer: simplify exported SVG by extracting styles

When a SVG document is exported from the 3D viewer, it now is optimized so that
all elements won't have style attributes anymore. Instead a class is added for
each unique style and this class is used instead on each corresponding element.

Fixes #820

fzadow/CATMAID,fzadow/CATMAID,fzadow/CATMAID,fzadow/CATMAID