Add a filter plugin to replace empty <th> elements with <td>

Problem/Motivation

According to the example provided on Tables with Multi-Level Headers empty <th> should be replaced by <td> elements:

In the following code snippet, the headers for the header cells themselves have been set to an empty cell (with the id “blank”). This will prevent some assistive technologies from reading a header for those heading cells.

Steps to reproduce

1. Use CKEditor to add a table to a text field.
2. Leave one or more header elements empty.
3. View the page source.
4. Notice that the empty elements are <th>&nbsp;</th>.

Proposed resolution

• Add a text filter that replaces empty <th> elements with <td>.
• All attributes from the original <th>, except for the scope attribute, should be transferred to the new <td> element.
• Besides <th> elements containing no text, also   and similar cases should be considered empty.
• The new text filter is meant to run after the existing table_header_scope_attribute filter; otherwise, automatically determining the scope will likely yield incorrect values. Validation should be added to the configuration form.
• Add functional, kernel, or unit tests to cover the new functionality.

Remaining tasks

1. Create the text filter that replaces empty <th> elements with <td>.
2. Add validation to ensure the correct ordering of the filters provided by this module.
3. Add/extend tests
4. Update README.md

User interface changes

None.

API changes

None.

Data model changes

None.