refactor!: Rename name to label and use value for label content

[mwcampbell]

I think the new property name and the new use of value on nodes with Role::Label makes sense because:

• For non-label nodes, their label is now determined by either a direct string in the label property, or the labelled_by relation. The word choice is more consistent.
• On a label itself, it doesn't make sense to talk about the "label of a label", but "value of a label" does make sense.
• For labels that have full text support via TextRun nodes, we can now automatically get the label's value from its descendant text runs, rather than having to set the value directly; this follows automatically from the existing support for constructing the value from text in accesskit_consumer::Node::value.

[DataTriny]

I'm on board with these changes, but I expect the transition to the value property to be painful. Maybe you should split this PR so it has its dedicated entry in the changelog, until we properly document everything?

Also, you're effectively creating a new kind of nodes which are labeled by their value, and I wonder if you should explicitly name them by adding a new method in the consumer crate. I currently have no idea how we'd name this class of nodes, but I don't really like having this check on roles in multiple places.

[mwcampbell]

I don't think I understand the benefit of splitting the PR.

[DataTriny]

It's a subtle change and I wouldn't be surprised if people don't set the value property on labels while migrating, so I'm just trying to see how to make this change more obvious. I think having a dedicated entry in the changelog would make it more noticeable.

[mwcampbell]

I don't like the extra churn that splitting the PR would cause in the code itself, just for one extra line in the change log.

[DataTriny]

Thanks for adding documentation. It doesn't mention that it also applies to Role::Image though. This actually make me question the purpose of this: if I hear "the value of an image", I won't think "text alternative". I'd say that the label property is more appropriate for this role. What was your reasoning for including the image role in this change?

[DataTriny]

Maybe this is to allow a rich text alternative?

[mwcampbell]

You're right about Role::Image. I fixed that, and also addressed your other suggestion.

[DataTriny]

Awesome, thanks! Please rebase and merge after that.