a11y: implement get_contents_at for Gtk.Text, Gtk.TextView, Gtk.Inscription
There are users of GetStringAtOffset, such as Orca.
Note: For Orca to actually make use of, it needs either GetAttributeRun as well, or orca!196 (merged) as a workaround. This also helps to trigger orca#477 (closed).
Edited by Markus Göllnitz