EPIC: `image-label` spec

[lubianat]

This is a parent issue / epic to connect issues in the image-label specification, which have different degrees of overlap and staleness

Ideally these should be summarized and put on some kind of roadmap

• Clarify difference between image-label properties and colors #79

• More clarifications of the "image-label" spec #105

• image-label: lost of original context #111

• Support for multi-channel labels #19

• Relax "labels must have colors" constraint #338

• "image-label" not camel case #109

• Resolving misc issues and ambiguities in labels section #170