Improve accessibility of ComboBox widgets

[DataTriny]

This improves how ComboBox widgets are represented in the accessibility tree so that they can be interacted with using assistive technologies. The "expanded state" is still unusable for now but since the value is properly reported, arrow keys can already be used to pick a choice without opening the popup.

The AccessKit platform adapters currently don't expose the Node::is_expanded property to the various accessibility layers, but this is expected to ship in the next release or so. Testing was performed with a branch of mine.

Changelog

• Improved accessibility of ComboBox widgets
• Added accessible-expandable and accessible-expanded properties, as well as accessible-action-expand.