Add a feature to TableOperations to allow table heading cells <th> to be defined

[trem]

Hi, I was wondering if the table plugin is likely to be extended to include options that allow the user to define table headers within the Insert table dialog box so that there can be <TH> tags in a table.

These are an accessibility requirement for tables as users with screen readers find it extremely difficult to make sense of tables without them.

(if you add them in manually via html, the product actually moves them inside the Table body tags)

[gogo]

dup to #640