Allow table cells to be specified as Header cells, to render as <th> instead of <td>

Very often theme CSS files will have a distinct rendering style for header (“<th>”) vs. regular (“<th>”) cells. We need a way to create, or at least mark existing cells as, header cells to make it easy for end-users to take advantage of this without having to hand-edit the generated XHTML.

Also, for header cells, there must be a distinction between Header for Row and Header for Column, which would add “scope="row"” or “scope="col"” attributes, respectively, to the generated “<th>” tag.

I recommend a set of radio buttons that would appear in a Format Table Cell(s) button (along with other options for formatting table cells, such as horizontal and vertical alignment [setting actual alignment options on the table cells or rows, not to be confused with wrapping the cell content in a “<v>” or “<p>” with “align=""” attributes]):

• Content Cell (<td>)
• Column Header Cell (<th scope="col">)
• Row Header Cell (<th scope="row">)