CadranSwatch

A few colours shown in an icon's place, the size of a row's icon square (cadran::List::row): a palette's in its picker (ui::pick()'s swatches). A rounded square of background, ring as a border inside it, motif (a CadranIcon's name; "sparkle" when it has none or an unknown one) in ink; around it a hairline in the theme's cadran/line, so that a dark swatch still shows on a dark sheet. With a picture (a theme's preview), that picture instead, cover-fitted in the rounded square with the hairline: decoded on the decode thread when the swatch is made (ThemeImages), the colours shown until it is ready, or when it cannot be read; freed with the swatch.