Add a script to automatically generate SVG math images from a LyX template.

This is an adaptation of Georg's generate_symbols_images script, but it uses dvisvgm instead of dvipng.
A waggonload of SVG math icons.

These are automatically generated via the generate_symbols_svg script. Some were modified manually afterwards.

These certainly need some manual care, and a number of icons are missing (cannot be created automatically). But it is a start.