.ui: Marking widgets which don't need accessibility labelling

[Samuel Thibault <sthibault hypra fr>]

Hello,

As part of implementing an accessibility non-regression check tool, we
want to make sure that widgets have proper labelling. E.g. GtkEntry
basically always need a labelled-by relation. Some other widgets don't
necessarily need one, but very often need one (such as radio buttons),
so we want to emit a warning if there is none, unless it was marked as
not needing one.

Conversely, labels are most often introduced for a reason, and they
should thus almost always have a label-for relation, except for
exceptions which need to be marked as such.

The question is then how we should mark them. Perhaps we could use e.g.

    <accessibility>
      <relation type="no-labelled-by" />
    </accessibility>

for widgets, and

    <accessibility>
      <relation type="no-label-for" />
    </accessibility>

for labels?

Samuel