Is your feature request related to a problem? Please describe.

Problem: Cannot create a table with row header cells; can create a table with empty header cells; tables not differentiated between presentational and data; captions use the wrong semantics.

Issues:

• It shouldn't be an option to leave header cells empty, or adding content should be prompted
• Tables without any header cells should be marked as role="presentation"
• The figure/figcaption implementation is a strange choice, given that there is a standard caption element (see comment on 15554)

See also #15283 for prior work towards making the table block accessible.

Describe the solution you'd like

• Should be able to add a header column
• If no headers added, table should be presumed to be presentational
• Captions should use 'caption' element
• If header cells are present and empty, user should be prompted to complete them or the cells should be reverted to .