clear font data

Clear different kinds of data in the current font.

RF3 RoboFont 3 code which still works in RoboFont 4. Not updated to the new RoboFont 4 APIs yet.

guidelines
delete all font-level guidelines
groups
clear all groups
kerning
clear all kerning pairs
features
clear all OpenType features
stems
clear all vertical and horizontal stems
blue values
clear all blue values
layers
delete all layers in the font
template glyphs
delete all template glyphs
mark colors
clear mark colors in all glyphs