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