refactor!: Clean up table roles and properties

[mwcampbell]

The roles and properties that I copied from Chromium are complicated compared to the ARIA standard, and the meanings of the various properties were never documented. I think that if the ARIA roles, properties, and (lack of) relations are good enough for both GTK 4 and ARIA itself, they're probably good enough for us, too. I'm quite sure we don't need the Column role; it doesn't even have a proper mapping on two of the three platforms. As for the old table_header, table_row_header, and table_column_header relations, I guess the AT (or in our case, maybe the consumer library) should scan for the headers rather than requiring the toolkit to specify such relations; there are certainly no such relations in ARIA.

The new row_index_text and column_index_text properties are based on ARIA properties that are also in GTK 4, that we didn't previously have.

[mwcampbell]

Oh, and I dropped the table_ prefix from the property names because, as with hierarchical_level, long names are just more cumbersome.

[DataTriny]

I fixed merge conflicts from #390. I'll merge now.