[dialog] Review usage of text content in footer.

[Westbrook]

Expected Behaviour

There is clarify as to whether text content belongs in the footer of a dialog and it is styled appropariately

Actual Behaviour

[Westbrook]

Created adobe/spectrum-css#1203 to track.

There in I propose that the footer be the same color as the content, and you can preview this approach in https://westbrook-dialog-footer--spectrum-web-components.netlify.app/components/dialog-wrapper