Replace autogenerated icons for the math toolbar

Replace autogenerated icons for the math toolbar

The autogenerated icons are in most cases ugly, do not conform to the
style of the other icons, and do not respect relative dimensions and
positions. The math icons have a vertical dimension of 19 pixel, defining
a grid that has to be respected for obtaining correct alignment.
The only autogenerated icons that remain are those of the Misc (extra)
toolbar. I plan to also replace them in the near future.