Suggest icon colours for every background kind

The picker next to the background controls used to offer a flat list of swatches for a solid fill. It now derives shades and analogous, complementary or triadic colours from the most prominent hue in the artwork, and those colours apply to gradients and blobs as well — one click fills the gradient stops or recolours the blobs.

Harmonies build on a colour that actually has a hue, so an icon on a black tile still gets a useful complement instead of a grey no-op.