Get/view/edit kerning

[justvanrossum]

• Read/write kerning (designspace/fontra) #1502
• googlefonts/fontra-glyphs/issues/51
• View kerning in canvas #2135
• Visualize kerning in canvas #2139
• Basic kerning editing tool for canvas #2151
• [kerning] Edit kern group info #2184
• https://github.com/googlefonts/fontra-glyphs/issues/98
• Add UI for group/class kerning exceptions #2204

Followups:

• Read kerning from TTF/OTF #2137
• Add kerning sidebar #2211