Colour labels reached `versions.label` only from an XMP sidecar: nothing
in the catalog could set one, clear one, or read it back alongside the
stars, so there was nothing for an interface to call.
`set_label` and `set_label_many` write it the way ratings are written,
the bulk form in one transaction so a key over a selection is one commit.
`toggled_label` holds Lightroom's rule for a label key: it clears only
when every image already carries that label, and otherwise sets it on all
of them, so a half-red selection comes out red rather than inverted.
`Judgement` carries the label, so the grid's one window query brings it
with the stars, and `label_histogram` counts each label in one grouped
statement for the filter chips. A label does not make a frame "judged":
it is a pile of the photographer's own, not a cull decision.
The doc comment for `default_version_id` had been stranded above
`label_code` when that was inserted; it is back on its function.
dtourolle
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