Pane titles should be in TH (rather than TR) tags

Currently, we put the pane titles in plain old TR tags with just a CSS class to differentiate them from the pane contents. However, since conceptually, the pane title is the "header" for the content, they should really be in TH tags.

However, since we'd end up with multiple TH tags in a single row, for this to be totally semantically correct, we should add explicit headers references to each value cell pointing to its title cell.

The main question is how screenreaders will handle that in the real world. If it's better or at least no worse than what we have with TR tags, I think we should make the change because it's more semantically correct.