a11y: AT-SPI TableCell methods GetRowHeaderCells and GetColumnHeaderCells not implemented

While it's currently not documented in its XML specification, the AT-SPI TableCell interface has methods to retrieve the row and column headers of a cell (GetRowHeaderCells, GetColumnHeaderCells), s. https://lazka.github.io/pgi-docs/Atspi-2.0/classes/TableCell.html#Atspi.TableCell.get_row_header_cells .

The Orca screen reader for Linux makes use of these since Orca commit https://gitlab.gnome.org/GNOME/orca/-/commit/ac2c4470ff4401b53b6de48e12ea5a0a6347897c ("Prefer table cell interface for getting row and column headers"), which results in issues with the qt6 variant of LibreOffice.

Sample steps to demonstrate the issue using a Python script:

1) open the attached sample document table_headers.odt
 in a current development version of LibreOffice using the qt6 variant (I'm using LibreOffice git master as of commit 84183c84d86456e3c311b35150b1fc9b63a85561)

2) run the attached Python (pyatspi) script pyatspi-script-print-tablecell-header-cells.py and look at the output:

$ ./pyatspi-script-print-tablecell-header-cells.py 
row header cells: [], column header cells: [] 

Actual result:

• the program takes a while to finish (because of timeouts)
• No header cells are printed

Expected result:

• The script should finish faster and print column header cells.