Provide UI for “Mute kerning”

[LettError]

A source needs to be able to have its kerning muted. The designspace file and the objects support it, <kerning mute="1"/>

It could be added as part of the muted glyphs popover in the sources panel

Alternatively, we could consider adding a This is a support checkbox in the sources list. That would need to be saved somehow and I don't have a proposal for that just yet. Maybe something like: <source filename="bold.ufo" name="source_1" support="1">

That might confuse the designspace readers.

But, suppose that has an answer, if we know a source is a support, it would be possible to generate previews for glyphs in this source that show how much the shape differs from a straight interpolation for that location.

• See https://github.com/LettError/ufoProcessor/issues/74
• See https://github.com/fonttools/fonttools/issues/3929