a11y: Add GtkAccessibleText interface
The AccessibleText interface is meant to be implemented by widgets and other accessible objects that expose selectable, navigatable, or rich text to assistive technologies.
This kind of text is not covered by the plain accessible name and description, as it contains things like a caret, or text attributes.
This commit adds a stub GtkAccessibleText with its basic virtual functions; the interface will be implemented by widgets like GtkLabel, GtkInscription, GtkText, and GtkTextView. A further commit will ensure that the AT-SPI implementation will convert from GTK to AT-SPI through a generic (internal API); and, finally, we'll remove the widget type checks in the AT-SPI implementation of GtkATContext, and only check for GtkAccessibleText.
Fixes: #5912 (closed)