Consider option for disabled buttons

[ebruchez]

Following #4210.

When a button link is disabled, it receives focus, which is sent to the server. The server updates the focus to the previously-focused item because it doesn't accept the focus on it.

Q: Should we accept keyboard focus on disabled buttons?

There might be more than one answer (see this discussion). A disabled button in a navigation, like wizard navigation, should probably support receiving focus. But in other cases, probalby not.