Add GtkAccessibleText interface for text widgets
Now that GtkAccessible
is public, we're going to need a public interface for widgets that provide textual data.
Implementation details:
- AT-SPI: https://gnome.pages.gitlab.gnome.org/at-spi2-core/devel-docs/doc-org.a11y.atspi.Text.html
- UIAutomation: https://learn.microsoft.com/en-us/dotnet/api/system.windows.automation.provider.itextprovider?view=windowsdesktop-7.0
- NSAccessibilityProtocol: https://developer.apple.com/documentation/appkit/nsaccessibilityprotocol#1674786
See also: vte#2614