feat: Support inherited flags

[mwcampbell]

So far this is just for the "italic" flag. We can decide later if this makes sense for other flags too.

This also assumes that if a flag is set on a parent, it makes sense for it to be implicitly set on all descendants too. For text attributes like italic style, that are typically set on individual runs anyway, I think this makes sense.

Closes #657

[DataTriny]

Making hidden an inherited flag would solve #657 but I'm not sure if that is a good idea, given that this would impact all adapters.

[mwcampbell]

To fully solve #657, would we also need to somehow modify Tree to fire node_updated on all descendants if the direct value of is_hidden changed on a node?

[DataTriny]

That would make the AT-SPI adapter simpler, but we can still add or remove the descendants like we already do to some extents. What concerns me is that the most frequent path (where a node has no hidden ancestor) is the most expensive as we always have to go down to the root node.

Anyway, for correctness and completeness, it would make sense to me to call node_updated on descendants when an inherited property changes. Have you looked into how text attributes are exposed on other platforms to know if we would need this?

[mwcampbell]

I'm willing to try making is_hidden inherited and see if there's a noticeable performance hit in practice.

[DataTriny]

Looks good to me. I've confirmed that this fixes #657.