ACTION-1519: Determine mapping of aria-colindex and aria-rowindex for ATK/AT-SPI

See: https://developer.gnome.org/atk/stable/AtkTableCell.html#atk-table-cell-get-position

Thus given a cell or cell-like object (i.e. something with a identifiable coordinates within a table or grid or similar parent container): The object with those coordinates should implement the AtkTableCell interface. It will need to inherit its rowindex from the parent row. Then the values should be exposed via atk_table_cell_get_position().

Related aside number 1, but not necessarily(?) something implementation-guide worthy, it appears that AT-SPI2 is slightly different wrt its interface:

* atspi_table_cell_get_column_index()
* atspi_table_cell_get_row_index()

But the principles are the same.

Related aside number 2: Seems we're missing some docs [1]. I've pinged the maintainer. But proof that the interface exists can be found in the code [2] in the meantime. (Yeah, I know. Sorry. The docs shall be updated soon.)

[1] https://developer.gnome.org/libatspi/2.14/api-index-full.html
[2] https://git.gnome.org/browse/at-spi2-core/tree/atspi/atspi-table-cell.h

While I had completed the task last month, I left this action open until I had platform API documentation I could point to (in particular, AT-SPI2 which had the API but not the docs see previous note).

Docs are now available without having to read the code:
https://developer.gnome.org/libatspi/2.15/libatspi-atspi-table-cell.html

Closing this action as completed.