Accessibility: Support text input widgets

[mwcampbell]

A major missing feature in Slint's accessibility support is exposure of text input widgets. I recognize that this is a major design challenge. For AccessKit (i.e. the winit backend), this requires a child AccessKit node for each span of text that has distinct formatting (at minimum, each physical line); see egui for a real-world example of how this is done. Let me know if you need help with this.